More than 2 player co-op (works really well). Also adjust amount of Terrorists in T-Hunt:
First you can keep the bots in a corner and play with your buddies then when a event for them is needed they will just teleport once you press the action button of the event are needed for, then tell them to get back in there corner. :P
You can play with up to 16 players but that would be overkill. I'd suggest 4-8.
You'll have to make a free account but check out TeknoR6Vegas2 3.0
<-- Only the host will need to do this.
Also a handy GUI to use with it too: http://teknogods.com/phpbb/viewtopic.php?p=32489#p32489
I would also suggest making the bot opponents a bit more rough for the added players. The best mod I have found so far is Frosty mod
<-- All players need this.
For playing online after May as GameSpy is shutting down you'll have to use an online LAN program like Evolve
<-- Personal favorite. Simple and works great.
or Tunngle or GameRanger. Don't bother with Hamachi.