To navigate to the Settings page, navigate to your User Name in the top right hand corner of the screen, then select the drop down arrow next to your User Name, select Settings from the drop down menu.
Once you’ve opened the Settings page, select User Groups from the options on the left.
From here you want to select the User Group you want to Add/Remove a User from. A popup window will appear with all the User Groups details, find the User amongst the list you wish to remove and select the REMOVE button next to their name. Then hit the UPDATE button at the bottom of the pop up window.
If you want to add a User to a User Group, select the drop down menu underneath the Members header, select the User you wish the add, and then hit the ADD button next to the drop down menu. Once, you've added all the Users you wish to add, hit the UPDATE button at the bottom of the pop up window.