Pretty much:
1. Select 'view' on which every PUG lobby you'd like to join.
2. Click 'join pug' (green font) button on whichever lobby you choose.
3. Wait for 10 people to join the lobby.
4. When 10 people join, the page will prompt you to 'ready up', click this.
5. Add someone from the PUG lobby, so you can join the lobby where the game is to be played. Perhaps add the person who created the PUG if they are in the lobby still, otherwise check out the chat, near the top of the page and see if someone has written that they'll host.
6. Join the lobby, in-game.
7. Play and enjoy the game. Most PUGs are Search and Destroy, and are to a custom ruleset, just to let you know. According to the rules, once in a PUG, you have to play it out, all the way until it is finished (2 halves), so make sure you have around 30-40 minutes free, because PUGs average at around that amount of time.
Hope this helps.
Cheers,
Sketch.
Posted on Friday, 21st October 2011