You must be the device owner to add users.
Go to Settings > System > Advanced > Multiple users. If needed, turn it on 

From your owner profile, you control whether users can call and send texts with your phone:
Go to Settings > System > Advanced > Multiple users.
Or from quick settings, tap 

You can remove yourself as a user. You must be the device owner to remove other users.
Go to Settings > System > Advanced > Multiple users.
Or from quick settings, tap 
Do one of the following:

