+1 to think in terms of audiences.
At our university we have 5 types of audiences:
- basic editors: they can edit pages but need help to structure their web site (+/- 300 people)
- advanced editors: they can structure their website, add collections and create PloneFormGen forms (+/- 50 people)
- ttw developers / integrators (scientists and "basic php developers"): they can customize Plone with a little CSS, add some jQuery functions and make little Python scripts TTW (+/- 20)
- developers : they can build Plone Product (2)
- system administrator: he installs Plone on the servers (1)