Well, if you're in the position of making tryouts, such as the leader of a clan, then this is pretty much how:
1) Make sure you have people who want to join, i.e. applicants. Get their MSNs, Xfires, whatever, so you can contact them
2) Decide how you will try them out. This could simply be talking to them and seeing if they're people who you would like to play with in a clan, or testing their skill and if they are good enough, letting them in.
3) Decide on a date. Let all your applicants know when it is, where, which server, etc.
4) On the day, book that server and let the tryouts commence
