The following lists some possible roles:
Admins / core users on all sites
Basically the people on [pox].
Wiki users on the main site
Users which want to contribute to a Wiki-like division of the main site.
Commenters on the main site
Users which want to comment on some content in the main site.
Journal readers on the journal site
Readers of journal articles who want to comment articles or configure the site to their own personal needs - like fiddling with ranking reports.
Editorial board on the journal site
The editorial board managing the journal.
Reviewers on the journal site
The reviewers working on single journal articles.