Join requests filter into the People tool. When join requests appear, you will see the user's name, message (if they wrote one), and two options: Approve & Reject. You can also click on the user's name to view their profile. The profile pop-up provides an additional Message option if you want to contact the user directly.
Approving a join request will instantaneously add the user to your portal. They will be notified of the approval. Click here if you would like to learn more about customizing your portal's join options.