Users can join groups, although I haven't tried, maybe they a user cab join more than one group so you can add any registered users to any group by making them a member of that group so they will be visible to multiple groups.

Elen, in this scenario, what you want to do is use the "Organizations" feature fo b2evolution. Add all professionnals to an organization called "Professionnals" and then you can display that organization like this: http://b2evolution.net/man/organization-members-widget