Multisite - subsite administrator

Dear Support,

I have been building a few sub-sites using multi-site. Now I have finished building one of them and want to pass the subsite to the new person who will admin the site.

I want to create a subsite A administrator, how can I do it? I tried to create ros1 in subsite A as administrator, but ros1 can also go into the mainsite and other subsites, B and C ... Why? How can I specify ros1 is the administrator of subsite A and just a normal "subscriber" in the main site, and subsite B and C?......

Also, an editor in site A should be just a subscriber of any other site in the network, how do I set it up?

I've "Join My Multisite" activated, is this causing the problem?

Please help.

Regards