Then, sure, we have to look at performance, infact, knowing about User Groups limits (73) described here https://groups.google.com/forum/#!searchin/joomla-dev-cms/User$20group$20limit/joomla-dev-cms/xpDLD_kOv6c/IPx4Lf7jv3kJ
(I'm not a progrmmer, so I don't understand if the problem is solved and/or why the discussion has been abandoned on November 2012 ?!?), I posted the question about "Why creating a Multisite the users are automatically generated?"
In an efficient system would be very appreciate the way to choose which User Gorup generate and which not (during Multisite creation), instead of simply choose from one of the three preset: Basic/Standard/Advanced http://www.seblod.com/v2/documentation/seblod-2x/multi-sites/1832-site-types.html
After that, I repeat, I'm not a programmer, but I think that creation is a need for the Seblod multisite feature, create a User Group and/or an ACL is impossible without have, minimum, a Public access for the specific Multisite domain yet created from the automatic Seblod process.
May be, one quick solution (about doing the minimun User Groups as possible), would be add a level before "Basic" site type, used to manage simple site which need only to display a front-end and not to have some administrator(s). If it's selected, SEBLOD will create 2 just the Public group and a Public access level.
...Or create an Administrator group (like "Basic" site do) is a need ?!?