Before meeting Danny Wallace I helped an old lady off a bus, left a bouquet of flowers by a stranger’s front door and gave away both my umbrella and my travelcard. Why? Because Danny Wallace is the leader of a 3,000-strong benign cult - he prefers the term "collective" - called Join Me, whose members are required to perform an act of kindness for a stranger every Friday. I thought I should practise what he preaches.

"I became a cult leader entirely by accident," says Wallace, who is 26 and a former BBC comedy producer. As he explains in his new book, Join Me, Wallace discovered last year that his late, Swiss great-uncle had once started a commune, which only three people had joined.

So he set out to see if he could equal that total by placing an advert in Loot, simply asking people to "join him" by sending him a passport photo.

Amazingly, a man called Chris Jones sent in his picture, and persuaded his flatmate to "join" Wallace, too. Wallace set up a website, www.join-me.co.uk, and more photos poured in.

As the number of "joinees" approached triple figures, Wallace realised he should give his disciples a purpose, and the concept of "Good Fridays" was born.

The movement spread quickly to Belgium and Norway, and pictures have even started to arrive from countries as far afield as the US and Australia.

Wallace has received congratulatory letters from Tony Blair and Prince Charles, and an endorsement by daytime royalty Richard and Judy generated more joinees, who gleefully report their good deeds on the website.

"Join Me is a genuinely good thing," says Wallace. "It's not going to change the world, but it does make it a little bit better."

The classic Good Friday deed is to buy a lonely old man a pint in a pub, present it to him, and then dash off.

"The idea appeals to people because it's the same mentality as playing a prank on someone, only you're doing something nice for them," says Wallace.

Very occasionally, the good deeds go wrong. Wallace was impressed when two of his joinees donated £38 for a train fare to a stranded motorist calling himself Raymond Price, but subsequently discovered the 59-year-old was a con artist who had been scamming good-hearted people since the 1960s.

	'Wallace realised he should give his disciples a purpose, and the concept of Good Fridays was born'

Undaunted, Wallace set up a fund soliciting donations for the man, which he will receive if he ever resurfaces - providing he promises solemnly to go straight.

I tell Wallace about my good deeds, particularly the look of delight on the face of the woman to whom I gave my umbrella.

"The lovely thing is that she will tell all her friends that someone gave her an umbrella just in case it started raining," he says.

He is less sure about the bouquet on the stranger's doorstep. "People might think there had been a car accident there, or a bereavement," he observes.

Together, to test the reaction to the classic "unsolicited pint" gambit, we buy a round of drinks for a group of people sitting outside a south London pub then take our own pints to another table. After some initial suspicion, they toast us.

"At first I thought it was a peculiar thing to do," says video director Chris D'Adda, 32, when I talk to him a few minutes later.

"But when you just went and sat at another table I was happy to drink it."

Emily Hudd, aged 26 and a TV producer, adds: "If I was here on my own I wouldn't have accepted a drink, but since you bought them for our whole group it was great."

Wallace now has another job with the BBC "thinking up ideas for comedy programmes", but Join Me now takes up much of his leisure time. It is not his first such stunt. In the past, Wallace teamed up with his flatmate, comedian Dave Gorman, for the TV series and book, Are You Dave Gorman?, in which they tracked down people called Dave Gorman all over the world.

Wallace also once tried to plan a holiday with his Norwegian girlfriend, Hanne, which involved visiting twinned towns, such as Birmingham and Milan, but she kiboshed the idea. And though Join Me may be Wallace’s most successful enterprise - morally and numerically if not financially - it also brought about the premature end of that particular relationship.

"I kept it secret from Hanne, and if anyone is thinking about starting a cult, my advice would be to tell your girlfriend about it," says Wallace.

"I always thought being a cult leader would involve me wearing robes and lots of women stroking my leg, but it’s been a dead loss on the relationship front."

As if on cue, my wife phones to say that we are late for a dinner party, and I realised I was stranded in the rain without a travelcard or an umbrella.

Maybe I won't join Danny Wallace's cult just yet, then.

• Join Me, published by EburyPress, priced £7.99 is out now.