How to assign site membership
You can set a site so that both server and site administrators can add or remove users on that site and change their site roles. At the All Sites level, server administrators can manage the sites existing users can sign in to (site membership), and their site role on each.
In the site menu, click Manage All Sites, and then click Users.
Select one or more users, and then select Actions > Site Membership.
Select one or more sites, and a role for each site, and then click Save.