How to get involved in fringe theatre
Well this is a no-brainer - get on Casting Call Pro! And no, I'm not on commission from them. It’s just that the huge majority of fringe plays are cast through them, and the vast majority of my fringe work is through Casting Call Pro, and so for fringe theatre that is unpaid/profit share/low paid, it is in my experience the best site to be a member of.
One should also be on Spotlight as well, as now and again jobs get posted there for the Finborough Theatre, Southwark Playhouse and other prestige London fringe venues that may well get you noticed and paid.
Other casting services which may work are Starnow (although I haven’t used them), and Dramanic (which I highly recommend - they tend to provide information about regional and professional theatre, although some fringe information appears from time to time).
Further to this, a good idea is to scan the websites of the fringe theatres you are interested in, and if there is a play/character you are interested in, contact them showing your interest. If nothing else, this is a good way of creating/maintaining contacts in the fringe, and more often than not you should get a reply. Look to see who is producing the play if it is not the venue itself, and then see if there’s a website for the production company. If not there might be a Facebook page, Twitter feed, Linkedin page for the director, or some other way of contacting them. Only give up if you feel you have exhausted ALL ways of getting in contact, and this includes phoning the venue or production company as well. I have phoned professional theatre companies many times, and people are always willing to provide information and help you out. With fringe companies, I can only assume they would be even more willing, as it is rarer for them to be approached this way.
One final piece of advice would be to check out Remotegoat or the Now Performing page of Casting Call Pro, and check out what’s on at the moment and what you are interested in seeing and what kind of work you would like to be a part of. Then attend a performance and see if there is anyone to talk to afterwards, be it the actors, director or the writer (the director will always be there for the first few performances of the show). Express interest in working with them, and if you have a CV, give it to them. If not, make sure you get in contact the next day, and if you don’t receive a reply, follow it up. People want to work with people who show perseverance, and before you know it you’ll have people swamping you to work with you, not the other way around.