Roles based security

I have a web site that has a membership base of about 50 000 people.
These people are part of a union.
They can login to the current site and have access to “restricted” areas that the general public cannot view.

1. Can wordpress 2.8 installed on a shared hosting account cope with a membership base of 50 000 users ?
2. Can I apply role based security to these members so that a section of the front end of the web site is accessible once logged in (but not accessible to the general public) ?

Any advice is appreciated.