To add an user to a tenant you have to go to the users account settings and click on the Tenants menu on the left side.In this menu you can click the Add to a tenant button and select the tenants where the user should be a member.
To add a group to a tenant you have to go to the group settings and click on the Tenants menu on the left side.In this menu you can click the Add to a tenant button and select the tenants where the group should be a member.