You can view and manage the people in a Group by clicking the View / Manage button beside the avatars on the group overview page.
(Note you can also view the people in a group from a discussion page, but you won't be able to add or remove them from here).
You will see a list of people in the group together with their online status. To add or remove one or more people, select Manage People and use the checkboxes to select or deselect various members.
It is important to click Confirm once you have selected the people to add/remove.
See also: Invite people to a Space