Sure thing! First go to the
Join a Game section. Now you will see a list of game lobbies people plan on hosting. If you have set your timezone up in your
preferences then all of the times will show up according to your timezone. Next thing you have to do is click on a lobby that you want to join (make sure you check the time & the date). This will bring you to the thread of that custom lobby. Once there, you will see a little box in the top right with the option to click 'Yes', 'Maybe', and enter your gamertag. If you want to join this custom game just enter your gamertag, click yes and press 'Post'.
You have now successfully RSVP'd for the game. Once the time for the game comes around you might want to send a message over Xbox Live to the host (whoever started the thread) so it'll be easier for him to invite you.
Hope you understood that all! If you need any clarification just let me know!