If you are familiar with rails (although it's worth learning if only for this), then look into http://wiki.github.com/stffn/declarative_authorization

I use it in 99% of all my projects to implement just that...

Thanks. The plug-in description (which includes RBAC) looks like it might be useful.

I did toy with rails while trying to choosing a framework over the past two months - but chose to develop using Django. Is there an equivalent based on Django/Python?

That is an extremely broad question, and the answer depends on your application.

I suggest you outline what type of application you're building and related requirements, and post that to http://stackoverflow.com

Thanks for the tip. The question is broad because I am looking for guidance how to even get started tackling the problem.

The application will allow buyers to complex RFPs online & sellers to respond to them - a collaboration. I will need access control to allow users within and across companies to collaborate with appropriate restrictions.