Start by saying hi to your group in the group chat. Just a simple "hey guys, just joined and looking forward to raiding with you!" is fine.
Next, take a look at any upcoming games in your group. If you see one you like, click the join button. Keep track of when it starts, because you want to be online on your xbox/ psn 10 minutes in advance so the game creator can send you an xbox/ psn invite.
If you don't see any games that work for you, you can post your own game. Try to post at least 24 hours in advance for best results. It'll automatically email all members when you post a gaming session, so don't double post or create fake gaming sessions. If you don't want to get these emails, just click "my account" at the top, edit your profile, and check the box to stop getting email notifications.
You can also leave and join a group at anytime, so if you don't like this group click browse groups and find one you like better.