1. Log in to the admin center.
  2. In the left hand menu under "admin centers" and click on "sharepoint".
  3. Click on "Active Sites".
  4. Click on the site you want to add users to.
  5. Click on Membership.
  6. Depending if they are a user or admin click on the appropriate tab.
  7. Click on add members.
  8. Add the members and you are done.