1. The "User Management" is handled from the "Settings" tab on the left hand side management panel.
2. After going to the "Settings" page choose "User Management". Here you can view your organization structure, find a list of the users in your organization, view pending users and also invite users to teams (more info on that here).
3. Click "Users" to get a list of all the users in your teams.
4. To view and possibly manage an existing user click on the the user and the information will be shown on the right side of the screen. From this view you can change the user role and also delete the user.
5. If you wish to add the user to another team, click on the "Add To A Team" tab on the right hand side of the screen.
6. Choose the team you want to add the user to as well as the role the user will have in that team and lastly click "Confirm".
Note that when adding an existing organization user to another team, no invite will be sent. The user will just get access to that team that they have been added to.