User Management Button is a 'human figure' icon on the top right side of the main page.
Clicking through it, you could check all the users, that has the access to your base station. Each user is shown as its username and email address, so it is easy to distinguished.
You could delete any users you don't want to access your base station any more.
You could also check the Geofencing Status on each user, 'In' indicate user is inside Geofencing range, and 'Out' means user is outside Geofencing range. You could change the 'In' or 'Out' status by clicking the icon, if you find the user is NOT in the right Geofencing status.