If 300 football fans chant together and then one assaults a rival supporter, are they all responsible? If you're on a protest and someone commits a crime and you don't leave immediately, can you be held to account for the person's actions? That was the question put before Westminster Magistrates' Court as we, the first ten defendants in the trials of those arrested for staging a sit-in at Fortnum & Mason on 26 March 2011, faced our verdict. We were found guilty of aggravated trespass; nine of us were given a conditional discharge and order to pay costs of £1,000 each, while the tenth was also fined.

The prosecution was required to prove an act beyond ordinary trespass — which on its own is not a crime. In this case, it argued that the protesters demonstrated intent to intimidate. Michael Snow, the district judge, accepted in his sentencing that none of us had been personally intimidating towards staff and shoppers, but said that under the terms of "joint enterprise" we were responsible for the actions of other protesters.

For the first few days of the trial, prosecution witness after prosecution witness — staff, customers and police officers — explained that most of those inside the store were, in the words of the chief inspector on the scene, "sensible" and "non-violent". One key prosecution witness, when asked by the prosecution barrister if he had seen anyone inside the store doing anything he believed to be criminal, said: "No." The police officers co-ordinating the case held their heads in their hands.

There is some evidence that a small number of acts inside the store may have been intimidating. There is no evidence that any of us on trial was responsible for these. In fact, in the case of many defendants, no individual evidence has been presented at all, and in my own case the court was shown footage of me engaged in the intimidating act of . . . facilitating a meeting inside the shop. But the prosecution maintained that we were guilty because we didn't leave when the intimidating acts allegedly took place. We will find out if the high court agrees when we take the case to appeal.

In a sense, this sort of verdict has been waiting to happen. In the past, it was hard to go on a potentially civilly disobedient protest without first knowing each other and planning it together. But in the Internet Age, it is increasingly easy to read a tweet and just pitch up at a location along with strangers. Can you, in this situation, be accused of "joint enterprise" with everyone at the resulting protest, even though you have never previously met them? Should everyone at such a protest be held accountable for the actions of everyone else? The implications of a guilty verdict are pretty scary — in effect, the Crown Prosecution Service and District Judge Snow believe that the only evidence they need to convict you for protesting is that someone else at the protest did something illegal.

This rests on a ludicrous premise: that it is acceptable to drag through the courts a group of people whose only crime is to have attended a "sensible" protest. Aggravated trespass legislation was introduced in 1994 as an explicit attempt to criminalise certain types of protest. Yet even this dubious law wasn't written so broadly as to include any demonstration in a shop.

This new development is worrying. Perhaps more worrying, however, is the disparity between the Crown's enthusiasm in pursuing the case, compared to their complete failure to convict a single banker over the acts that led to the financial crisis of 2007-2008. We'll see them again in the high court.

Adam Ramsay blogs for Bright Green