It's almost exactly three years since I joined a re-enactment group. After discussing the hobby in the GPOYW thread, I thought some people on the forum might be interested in knowing how to get involved.
Firstly, choose your period. I do American Civil War because I have a great interest in the period. Why you might choose a particular impression is up to you. It might be as simple as the fact you've heard they are good bunch of fellows, or you think you'd look more attractive in rifleman green rather than as a redcoat. People join for different reasons, but an interest in the period helps, if only so you know who the gun should be pointing at.
Next, find a group. The web is a big help for this - just search for, say 'Crimean War re-enactment' and you should be able to find something. Your local WH Smith's might take 'Skirmish' magazine, which has a index of groups at the back and also carries articles and photos of events and impressions. The best way, however, is to attend an event as a spectator. Walk round the camps, watch the battle, and talk to the re-enactors (we don't bite). Get a feel for what the group does, and what the people are like. You'll want to see what level of authenticity they expect, what the groups does, if the group family-oriented or not, and if you feel you'd fit in. If you like what you see, ask about recruitment. Multi-period events are even more fun as you get to see plenty of groups across all periods rather than just one - many re-enactors use them to seek out new impressions.
The next part is the hardest: making contact. If you have a name or a contact, call or email, else follow what they say on their website. You should then find yourself invited along - they will provide you with most of the kit too, on the understanding that if you join up over your first year you will return it. Your training will start on the first day but they might send you some information sheets to be getting on with. Oh, and you will screw up. Repeatedly. It doesn't matter. We all did too, and continue to do so. Other members will chip-in with their advice, and don't be afraid to ask questions.
Due to the gun laws in this country you probably won't be handling a real gun on your first few outings, and even once you've obtained your forms the club might still have their own assessment scheme before letting you fire. Non-firers do tend to die first, sometimes repeatedly.
Things to consider:
Authenticity - this is the big one. You don't want to be a farb (someone who doesn't care how they look). Whilst it can't be helped that re-enactors tend to be older and fatter than the people they represent, look around their campsite. Are modern items kept out of sight? Are the uniforms and kit appropriate for the time and the person being portrayed?
Having said that, you might not want a totally authentic group. At the extreme end are those that will expect their members to be in the 1860s for the whole weekend, with late night guard details, forced marches, period conversations and proper rations. Then you get other groups, like mine, that seek to provide an authentic look during the public hours (I'm in the habit now of taking circuitous routes back to camp to avoid saluting officers), but in the evening the beers come out or we go to the pub, and we have a whale of a time, then collapse onto our modern campbeds and keep warm in four-season sleeping bags. It's a question of what you want to spend your time doing. Talking to a group's members and watching them 'at work' will help you decide if it's right for you.
Commitment - re-enacting is an expensive hobby. Whilst clubs will loan you gear, it is expected that you get your own. Custom-made uniforms are expensive, but thankfully there are always people selling old kit so keep your ears to the ground. Indeed, second-hand stuff is sometimes the better option: I am supposed to portray a gruff, dirty western federal infantryman but when I first got my uniform it was far too neat and tidy for the look I was aiming for. Moreover, as you get addicted to the hobby, as you will be, it will be be draining in terms of petrol.
Rank - if you enter a military-style organisation, you will start out as a private (or equivalent). But remember, authority in re-enactment groups is based on consent: if you believe an order is dangerous then you are under no obligation to follow it. Moreover, there is, in reality, only one rank - that of 're-enactor'. You portray a private, he is portraying a sergeant. The relationship Pvt Kern has with Sgt X is very different to my friendship with X. Do not join a military group because you think you'd make a great commander: if you think that, you should be trying for the real military. You are portraying people and roles from history, and every person re-enacting a private in the ranks is just as important and vital to the success of the hobby as the major. Indeed, some of the best re-enactors I know have never led a unit but concentrated on forever improving their impression.
Finally: have fun. Unlike those we portray, we know we are going home at the end of the weekend and are, most likely, good friends with the people on the other side of the musket. The cheers from the crowds after a successful performance always raise the spirits, as does having a member of the public talk to you about the period (it's the only time I normally get to pontificate on the American Civil War) and the kit. And, of course, you'll meet great new friends too, and sometimes get the privilege of shooting at them