As others have mentioned, finding an empty server and piling in with all your friends is one method, but in team game types it's unlikely that you'll all end up on the same team. Otherwise, your other option is parties.
Free-for-all Deathmatch has a max party size of 10 last I heard.
Co-op TDM against bots has a max size of 6, but that necessitates an empty server being available so you might have to wait for a little while.
Co-op Bot Destruction has a max party size of 4
All other modes have a max party size of 3
The small party sizes is to mitigate an organized party's ability to completely wreck a public game (this restriction wasn't always there, and it did get ugly sometimes), but it can definitely be annoying when you have a group you want to play with.
I hope that there comes a time when the game's polulation allows for parties and solo players to have completely separate queues so that bigger groups can play without ruining games for randoms.
Edited by DM30, 29 April 2015 - 03:38 PM.