If you're subscribed to a team plan (Group, Agency or Powerhouse), you can add and remove teammates from within the Manage people page. 

Access this section by selecting Manage people from the dropdown menu on your Project Dashboard. 

Find the user you'd like to remove and click the three dots next to their name. To remove them from the team, select Archive this user. 

You can add them again at any time by clicking the Archived people link. 

To restore their place on the team, select the level of access (full or restricted) that you’d like to give them.