All forms of collaboration require some sort of centralized collaboration tool.
In Drupal this can be implemented with "Multi-Sites", "Organic Groups (OG)", and "Domain Access". These solutions all have their strengths and weaknesses.
The main issue however is when to choose "Multi-Sites", "Organic Groups (OG)", or "Domain Access".
First a short overview of the relevant points:
"Organic Groups (OG)"
For the following reasons I prefer "multi-sites" with separate databases over "Organic Groups (OG)" and "Domain Access".
Depending on how fast or easy you need to set up, I would recommend the "domain access" module if you want to keep an external (sub-)domain structure with a single administration interface.
When an outwardly facing (sub-)domain structure is not needed, and a single (sub-)domain will suffice, the "Organic Groups (OG)" module family is advised.
The more all your "groups" are alike in theming, structure content, and the more users need to be able to get to all groups, the more attractive the single database and administration solutions ("Organic Groups (OG)" and "Domains") become.
In a "multi-site" with separate databases, these can be accomplished with the core Open Id and Aggregation modules, but the administrator/developer might need a higher level of familiarity with drupal.
Depending on your setup, a single administration interface is not such a hot item. Wether you have change theme settings on 5 different sites or change the theming settings for 5 different groups you still have to know what you are doing, and perform 5 different actions. Simultaneous login for administrators in 5 sites can easily be accomplished with most modern browsers or their add -ons.