Only the Super Admin can assign the Admin role. Go to Manage Teams > Roles & Permissions and select the team member you want to assign as Admin. Once assigned, the Admin will automatically have access to all groups across the organisation. Admins do not need to be assigned to specific groups individually.
Note: A user cannot hold both the Admin and Group Manager roles at the same time. If you assign Admin to an existing Group Manager, their Group Manager role will be removed and their group will revert to the Super Admin as the default manager.