Multiplayer is very simple, no special effort needed. Just click Multiplayer and enter the IP address there. We use Inspire's server which is 203.114.130.15 . Ensure you have set a player nickname or it will kick you if you use a default name. We are currently using version 4.08m. You can join whenever and play as much or as little as you like. The standard Inspire dog fighting sessions are on most nights (Mondays is most popular) but the server runs 24/7.
If you want to be involved in the semi-organised co-ops, you don't have to commit to anything. We generally run these on Fri and Sun evenings 8pm - 11pm ish. You can join and leave when you please though, you won't let the side down

Sometimes if there's only a few on Inspire someone will host a co-op for some teaming up against AI planes, which are actually not bad as far as AI generally goes.
We also use Inspire's Teamspeak IL2 channel. We do split into Red and Blue Teamspeak channels if it gets 'serious' but generally it's all good being in the same channel, lots of fun.
When one of us hosts a co-op we pass on the IP address in Teamspeak first, so meet there to get info about the co-ops, as it's hosted on a member's personal PC we need some way to give out the current IP address.
When on the server, type <help in All Chat to get a list of commands you can use to find out what planes/loadouts are flyable, what your targets are, your statistics etc.
Further info at
Kiwisim IL2 ForumMatt