Members can be added in multiple groups. They only have access to the content of groups they are a member of.
If you click 'Everybody' in the left sidebar you'll see a list of all people in your organisation:
From within a group, click on 'Members' in header menu, then hit 'Add Members':
Now you can add existing users to this group, or add a complete new member. When you add a new member, an email with login link will be sent automatically:
Administer user managers
You can add or remove a member as user manager: go to 'Everybody' and click on the drop down icon next to a user. From there you can add or remove the user manager role.
Remove members from group
In the header menu of a group, go to 'members' and click on a drop down of a user. From here you can remove someone from a group:
Go to 'Everybody' and click on 'Block user' in drop down of user card, see screenshot at 'Administer user managers' above.