The Site/User Manager in the Study Configuration will allow you to distribute users among sites and assign study roles. 

This opens lists of all users and sites.

The users displayed are the users in the User Manager who were enabled for the current study (read about that here). 

The sites displayed depend on which sites in the Site Manager have been enabled for the current study (read about that here).

Tap and hold to drag a user from the left side to the site where they need to be added. The Default Role option that is currently selected at the top will be the role that users get as you drag them to their site. Most users only need access to a single site.

The user count at each site can be seen in the Sites list. By tapping on a site, the user list and corresponding study roles can be seen at the bottom.

The user roles can also be changed at the bottom directly under their name.

This is true study management in your pocket!