If you've set up the application to allow people to register on their own, you can moderate those new registrations. (A system administrator enables both user registration and moderation for new registration requests.)
End User UI: Your Stuff > Items Awaiting Approval (admin profile's Pending Approvals tab)
When someone registers (usually by filling out the registration form defined by the system administrator), you'll get a chance to approve or decline their request. In the end user UI, click Your Stuff > Items Awaiting Approval. On the Pending Approvals tab of your profile, view the list of requests. You can click the person's name to view a profile page with the information they entered. Click Approve or Decline for each request. If you decline, you can enter your reason before finishing.