In Metatask, any user can be a member of multiple groups. As Administrator, you can include or exclude user from different groups. To do this, go to account drop-down menu, [Team name] settings, and choose tab "Users".
Click on the user name you need, and you'll see user details panel on the right. To edit the list of groups for this user, press link button "Edit". Then choose the groups you want to add from the drop-down list or exclude groups by pressing "Delete" button (the X) opposite the group you need to delete.