Click 'Users' in the top menu.

If necessary, use the search box to find the user you are looking for.

Click on the user options arrow.

On the drop-down menu, click 'Add User to Group'.

In the 'Add Users to Group' pop-up window, start typing in the name of the group you would like to add the user into, and select it from the drop-down list.

Click 'Save'.