The Project Users page looks like this when you first log into the application.
You can see all your users already assigned to this project, and the role they are assigned under. This is either Manager or Contributor.
If you would like to add more users, you can click the Add users link top right, to choose users already added to your company. You can also invite a user to join your company Tickd by clicking the Invite users (also top right) who is not yet currently subscribed. An email will be sent out notifying them they have been invited and asking them to log in with the credentials provided.