User management is managed from the "Settings" tab on the left hand side management panel.
1. Click on "Users" to get a list of all the users in your teams
2. Click on the the user you wish to add to another team, and the user information will be shown on the right side of the screen. Click on the "Add To Team" button to add the chosen user to another team
3. 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.