There's a control panel menu on the left side of the screen. At the bottom of this control panel, clikc on Group Memberships.
You should be presented with a list of groups available to join, and any groups to which you already belong. If you are a group leader, you should also see any pending requests to join the groups for which you are the leader. Click Review Join Requests to see the pending requests.
On the list of pending requests, you can either use the button at the top of the columns to set all pending requests to Accept, Deny, or Ignore. You can also set each request individually using the radio buttons.
Once you've selected who to accept or deny, click the Do Authorizations button.