Bruce,
It sounds like you are on the right track. Large projects are often broken into smaller groups so your 15 people might be broken up into 3 or 4 groups of 2-5 people along the lines of how you would normally break up a project. For example if this were a high-rise one team might be working on typical floor layout, another the skin, and another the core and yet a fourth group the entry level then there is the subterranean garages; you get the picture. Each group might have their own file which is cumulatively hotlinked into a main file.
The above might be a fairly complex building, but you will be able to make similar kinds of judgments on smaller projects. Layout out books can be be broken out as their own role so whomever is free can update layout books etc.
You are right to keep tasks such as attributes in the hands of the more experienced.
However you set it up, keep in mind how the staff will, or can. grow in their abilities. The more they are willing to learn about how to properly learn Archicad, the soon they can be trusted with modeling and other more critical areas.
Hopefully Link Ellis will chime in here; He's just setting up a large firm in Australia. He is experiencef doing this round the globe.
HTH
Erika
Architect, Consultant
MacBook Pro Retina, 15-inch Yosemite 2.8 GHz Intel Core i7 16 GB 1600 MHz DDR3
Mac OSX 10.11.1
AC5-18
Onuma System
"Implementing Successful Building Information Modeling"