A menu will pop up, where you can proceed to click on Teams under the Company headline:
Once in the Team Management page, select your team (for example London) in the list:
Once your team is selected, the list of users belonging to this team will be showing in the middle while the list of unassigned users will be showing in the right side:
Note that if you do not have any Users without team, this means that all of your users are already assigned to a team. If you wish to move a user from one team to another, you must then first remove that user for her initial team.
Lastly you can add a user to another team by clicking the Plus icon next to his name: