Image caption Nairobi's Westgate mall was badly damaged after the four-day siege by an al-Qaeda affiliate

Al-Qaeda's standards for membership have slipped. The organisation is admitting a new generation of members - and expanding its reach.

Osama Bin Laden did not want the Somali Islamist group al-Shabab to join the al-Qaeda network. He criticised their leaders in a letter that was found in Abbottabad after he was killed in 2011, implying that they imposed unduly harsh penalties on "those whose offences are ambiguous".

Al-Qaeda's new leader, Ayman al-Zawahiri, is less concerned with al-Shabab's shortcomings. Less than a year after Bin Laden's death, Zawahiri welcomed al-Shabab into the fold.

"He thought it would extend the reach," says Richard Barrett, the former co-ordinator of the al-Qaeda and Taliban Monitoring Team at the United Nations.

The induction of al-Shabab shows a new style of al-Qaeda leadership. Zawahiri and his cohorts are more accommodating - and also more ambitious in their scope - than their predecessors.

"They've franchised themselves out," says Daniel Green, a fellow at the Washington Institute for Near East Policy.

Image caption Zawahiri has been helping to expand the al-Qaeda franchise

Leah Farrell, a former Australian Federal Police intelligence analyst, says: "If you add in affiliates and franchises and branches, then al-Qaeda's bigger now than it ever has been."

The reason for the admission of new members is simple. "Vintage al-Qaeda," she says, describing Zawahiri and his cohorts, have not carried out a major attack on Westerners in years.

You get a lot of street cred - but you also know that with the designation comes your likely death Daniel Green, Washington Institute for Near East Policy

For that reason, al-Qaeda leaders are turning to other ways to maintain their place on the global stage - and are less strict about admissions requirements.

On 21 September, al-Shabab launched its attack on Nairobi's Westgate mall, killing more than 60 people. The brutal nature of the attack ensured that al-Qaeda would remain in the news for some time.

Meanwhile, another al-Qaeda-affiliated group has taken control of a border town in Syria. Its leaders are not the only ones who want to carry the al-Qaeda banner.

Militants in Indonesia and other countries have been trying to get the attention of Zawahiri. "They're essentially waving their little hands about and saying, 'Please can we join?'" says Farrell.

Militants want to join because they know their organisation will be transformed by their affiliation with al-Qaeda.

"What's in a name? A hell of a lot," says Barrett.

Image caption The US briefly closed consulates in Lahore and other cities because of al-Qaeda

For many militants, the name al-Qaeda conveys a sense of "purity", he explains. "It says that you're not corrupt and that you're ruthless." Membership in the club has other consequences, too.

"You get a lot of street cred," Green says. "But you also know that with the designation comes your likely death."

More than 1,600 militants in Pakistan have been killed by drones over the past nine years, according to a report by the New America Foundation, a think tank based in Washington.

Drone strikes have killed many of al-Qaeda's top leaders. Because of the looser standards for membership in the organisation - and the relatively broad definition of al-Qaeda in a 2011 White House report - al-Qaeda is bigger now than it ever has been.

The al-Qaeda franchise Al-Qaeda in Iraq: a Sunni extremist organisation

Al-Qaeda in the Islamic Maghreb: one of the best-armed groups in West Africa

Al-Qaeda in the Arabian Peninsula: its members have vowed to attack foreigners

Besides the "al-Qaeda core" - Zawahiri and his cohorts - the organisation includes a handful of affiliates, explained the American Enterprise Institute's Katherine Zimmerman in a September paper.

These affiliates include al-Qaeda in Iraq, al-Qaeda in the Islamic Maghreb, and al-Qaeda in the Arabian Peninsula, as well as al-Shabab.

In addition, al-Qaeda has supporters who do not have formal ties with the organisation.

"There are over 100,000 partisans all around the world who contribute to al-Qaeda's aims and subscribe to its ideology," wrote Noman Benotman and Jonathan Russell in a September paper for the Quilliam Foundation, a London-based counter-extremist think tank.

Zawahiri has made it easier for militant groups to become part of the al-Qaeda franchise. Yet joining the network still takes time - at least a year, explains Farrell.

Militant leaders try to keep their correspondence private. That may mean downloading a document on to a thumb drive and giving it to someone - who will then transport it to a courier in Pakistan.

Image caption Many Syrian rebels resent the presence of al-Qaeda-affiliated groups

"That person will have to get the document to Zawahiri," says Seth Jones, author of Hunting in the Shadows.

Once the militants have established contact, they will begin to work out a partnership.

"The dialogue starts with ideological issues," says Jones. They may talk about "the legitimacy of attacks or the goals of a specific group".

Once terms have been worked out, Zawahiri will announce that the militant organisation has become part of al-Qaeda.

Image caption Al-Shabab killed more than 60 people at the Westgate mall

Al-Qaeda remains notorious for its grisly attacks on subways, shopping malls and other places. Yet at the same time, the organisation continues to draw people into its fold.

Zawahiri says that Western authorities will never succeed in wiping them out, according to Barrett. Al-Qaeda is more than an organisation, he explains, "it is an idea".

"All the efforts have been about destroying the structure without dealing with why people join," says Barrett.

The power of al-Qaeda to attract new members testifies to its enduring quality - and shows the challenges Western officials face when trying to tamp down its threat.