I never contributed to an anti-gun group, but I think that's a very high standard to ask about. Very few people contribute to anti-gun groups. They tend to be funded by either by a very few rich individuals, or by a rich foundation (like the Joyce foundation) or group. I have had friends in the past who were rabid anti's in any discussion, but I don't know of any of them actually joining/supporting an anti-group.
On the other hand, just about every fiercely pro-gun person I know is a member of either NRA or GOA. Sometimes both.
Like many other's here, as a young man I was not concerned about gun rights and just kind of assumed they'd be whittled away as I got older and disappear. An outdated right, something unavoidable.
Then when Colorado started debating shall issue concealed carry in '98/'99 I got interested in the issues, and started reading the various writings and studies. That's when I realized the anti-groups were just plain anti-gun and pro-ban irregardless of the facts, and casually made things up or incited fear based on irrational viewpoints. When a group's whole base of arguments is made up of half truths and outright falifications, you've got to red flag the whole cause.
And while the NRA can certainly spin a study too, even if you reject the more outlandish pro-gun studies and just look at the studies one can assume to be neutral or even generic and not intended to support/dispute gun views, outlawing guns would NOT bring us down to a murder rate like is seen in England or one of those countries that the anti-gun groups keep pointing too. All it would do is disarm those of us who are willing to keep and use guns for self defense.
So I was never a card carrying anti-gun guy, but it's the anti-gun groups that turned me into a card carrying pro 2nd amendment guy.
Ironic, eh?