Yes, women can join the freemasons – the Co-Freemasons that is. But why would anyone want to?

On a leafy street in the London suburb of Surbiton, a big white sign welcomes visitors to a masonic lodge for "men and women". The lodge is an imposing Edwardian mansion, down the stairs of which comes a white-haired man offering his hand to shake, which is a bit hurried on a cold, wintry morning, but not particularly funny.

Julian Rees is a member of the International Order of Co-Freemasonry and he is keen to disprove the sense that it is a secret men-only society. The visit to Surbiton was arranged by a press officer after I called the rules on allowing women to join "complicated". Offering to carry my bag before he proffers a cup of tea, Rees explains that his order has welcomed women since its formation by feminist and socialist Annie Besant in 1892. Women now make up more than half of the Co-Freemasons estimated in the UK today.

Yet, as we climb the hexagonal staircase of the British HQ, filled with symbols and pictures of elaborately dressed masons, the presence of women doesn't detract from some of the bigger questions about the freemasonry, such as why a publicist is arranging meetings with a society best known for its secrecy. The answer lies in the fact that freemasonry in this country is in something of a crisis.

Apart from the important difference of the inclusion of women, the International Order of Co-Freemasonry, with talk of rituals, symbols and "the Craft", is identical to the better-known United Grand Lodge of England (UGLE) to the unitiated eye. And both are suffering from a declining and ageing membership base.

Although official numbers are hard to come by, most estimates suggest that there are some 6 million freemasons in the world and just over 300,000 in the UK. In comparison, 300 men and women belong to the UK arm of the Co-Freemasons. At its peak in the 1950s, there were five times as many Co-Freemasons, a rate of decline that many believe is echoed in the main branch.

At its postwar peak, membership of a fraternity that began as a sort of union for medieval stonemasons was boosted by returning armed service personnel as well as some of the most powerful men in the land. George VI, who died in 1952, is the last British king to be listed on the official UGLE website, which also includes Winston Churchill, an Archbishop of Canterbury and a surprisingly long list of celebrities from Nat King Cole to Peter Sellers. The royal connection continues today with the Duke of Kent, who is the current grand master of the UGLE.

Freemasons have long denied suggestions that it is a pernicious old boys' network, arguing that it is a sort of gentleman's club, concerned with moral and spiritual growth. Although in the UK the sense that freemasons are no longer the force they once were has given rise to jokes about suburban middle managers prone to rolling up their trouser legs and doing funny handshakes, there are signs elsewhere that membership confers preferment. The collapse of Propaganda Due or P2, an order that linked Silvio Berlusconi to the Italian central bank and the heads of all three secret services until it was closed down in the 1980s, did little to end suspicions.

Given this double whammy of conspiracy and mockery, it is no surprise that all parts of the fraternity are looking for a rebrand. Or the fact that Co-Freemasons want to disassociate themselves from the main branch, employ a PR company and launch a "recruitment drive" specifically aimed at attracting younger women.

Brian Roberts, a retired businessman who works "eight days a week" as the British grand commander, says that, by meeting the requirements of the Equalities Act at least, Co-Freemasonry "fits with the current age". With membership fees of £90 a year it is also "cheaper than most golf clubs". But everyone knows why you join golf clubs. Why would anyone want to become a freemason?

Nikki, Julian and Sandra are all Co-Freemasons. Photograph: Linda Nylind for the Guardian

A morning spent talking to four master masons makes an organisation that members still call a fraternity sound like church without the hymns. But Rees says, "It's dangerous to associate it with religion. We accept people with any or no religion. We follow a religious path outside religion." Some masons are atheists, he says, although they have to sign up to meetings which pray to a spiritual being, which seems a bit odd.

There has been a long and often bitter history of mistrust between organised religion and freemasonry. At its most benign it led to a spat between freemasons and the former Archbishop of Canterbury, Rowan Williams, when he suggested their beliefs were incompatible with Christianity. He then got into trouble for appointing a freemason to be bishop, thereby proving many avenues are open to freemasons but still closed to women.

But there are obvious similarities in the way adherents speak about their beliefs. Nikki Roberts, Brian's 31-year-old granddaughter, is held up as an example of the new kind of freemason with her Facebook page and media-friendly ways. Having dabbled in Buddhism, she gave up her job in the City before finding "stability" in Co-Freemasonry. She says the order adds "greater meaning" to her life.

But what about the signs and funny handshakes? These are apparently only used "if you need to prove yourself and you don't have your passport", says Rees. A passport? Disappointingly, it looks just like an aged travelcard with weird stamps inside.

In trying to explain freemasonry, Sandra Clarke, a businesswoman who comes up for the lodge's eight annual meetings from her home in the Cotswolds, says: "At the lowest individual level it's about practising the essentials of freemasonry every day. In that way freemasonry is no different from any other organisation with the added initiatory aspect and spiritual context." This secret initiation – of which little is known apart from the fact that new members are blindfolded – tends to arouse suspicions among outsiders. "It's not about hiding the location," says Rees. "It's so that he can look inward."

If somewhat vague on why people become freemasons, those I spoke to are clear on why they shouldn't. "If anybody wants to join to use it to gain preferential treatment in business they have completely the wrong idea," says Roberts. "It is a total myth." Clarke adds, "We would turn away people looking for personal material gain of some kind."

Turning people away seems to conflict with the idea of a recruitment drive that is drumming up business with a website, Facebook and Twitter accounts. Basically, anyone over the age of 18 can join, but not everyone is accepted. Why?

"Trust us, a lot of people do come along whom we subsequently find not to be suitable," says Roberts. "They have the wrong perspective, the wrong idea about who we are."

What makes someone "suitable"? Clarke says "It's not necessarily one particular thing, it's more we don't resonate with each other. They may have other ethos and values. They might be better off in a church, say. Or a business network." The others talk of making an "emotional, moral commitment" by commiting to the standards expected of freemasons, whatever they are. "We don't think we're better than other people," says Clarke, "but we do think we can make ourselves better."

Many freemasons such as Nikki grew up with family members in the fraternity, which also underlines the sense of freemasonry being an all-white club. Rees protests, describing "half" his central London lodge as "non-Caucasian".

Rees, who defected from the all-male side because of an argument over its lack of "spirituality", is also keen to stress that the differences go much further than the fact that the bigger male-only arm also has a much grander HQ in London's West End.

"The male order, much as they may deny it, is all about wearing more and more elaborate regalia and advancing to a higher rank. Male masonry is peopled by old grey beards, the aristocracy, major generals of the army, and they're nearly all male chauvinists."

Perhaps this differentiation is working. Since the recruitment drive launched last November the freemasons report growing interest in membership – mostly from women.

With the website and Facebook pages, freemasonry no longer seems as secret as it did. But why anyone – male or female – would want to join is still unclear.