Groups!?

it's easy for some of them.
you go to their home page, like this one
and then just click on the big green button.
poof, you are now part of a group :D

Groups are like "clans" in MMO - you join them, then you are a member of that group, there's nothing more to it.

You can join more than one group.

Most groups are dedicated to certain Steam games, to find people you can play with.

To join a group, find the group on Steam and join it if it's public - if it's a private group, you need to be invited before you can join.

btw: you are already in 9 groups, so why did you ask? :P

Here's a giveaway group you can join. It's an open group, so your chances of winning are much much lower than exclusive giveaway groups, but said chances are still higher than public giveaways. There's currently one giveaway up at the moment that I know of.

Edit: Also, never forget to Sync your account after joining a new group.

Just remember that while most groups are open for everyone, some groups are private and might have special requirements for joining them. If a group is private, NEVER start bluntly asking for an invite to join unless the group clearly allows that. Most groups don't take kindly to people for beg for an invite, and you could be blacklisted and permanently prevented from joining that group.