Well, unless you or one of your friends had been banned there shouldn;t be anything stopping you getting on a server. There's 3 reason you can't join:

1 (most Common) - The server is full, and has reached the maximum number of players allowed. Try another server.

2 - The server you're trying to join is running a modification that you don't have. Only join servers with a version of 1.00 (not the red ones).

3 - You (or your firends) have been banned from the server.

Hope that helps. If you are joining a server in order to play with friends, it's best to join a server than has a good 10 or so spare player slots, this way you can ensure that you all join. If there's three of you trying to get into a server with one slot left, only one you will get in.