In order to manage users you need to have the Manage Users permission. Check out the Reviewer roles / permissions article to see what roles have that permission. To get started first make sure you are inside of the correct project and click on settings icon.
This will bring you to the project setting section. To remove a user from your team you want to click into the Team Members tab. This will list all the active/pending team members that have access to the project. Click on the user you want to remove.
Hit the Remove from team button and that will bring up a dialog box to confirm the removal. Click yes and the user will be removed from the team.