LDAP Groups can give permissions to different sites.
When a user is in the Skyrim Group, they have access to the Skyrim Site, but not to the other sites.

2 comments
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Phil Cordier commented
Strong support for this idea. You should be able to configure an LDAP group to hold only those members that you wish to have access to a specific site. The functionality of the current group mechanism is not of much use, at least to me, otherwise.
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Simon Welling commented
I would suggest a separate group access settings page on blog level. So all the connection settings on network level, all extra (custom) settings on blog level.