Assigning one or multiple users to an existing department/team is done by navigating to Admin > Organisation > Departments. Locate the relevant department/team and click the icon to add users.
Users already assigned to this department appear in the overview. To Assign users, click the blue button and mark one or more relevant users to be assigned to this department/team.
Bonus info: Users can be assigned to multiple departments and teams at the same time.
Here you can learn How to Activate Users.