Hey @Joris_B, plans are handled at the team-level not account-level.
So for example your company could set up a team (company name) and sign up for the professional plan. This would then allow for unlimited number of projects and files/pages. Then you would add the team (company) to the team (Figma) as either editors or viewers. Each editor would be charged where as viewers are free.
Note again that it doesn’t matter if an editor/viewer has an existing plan of any type as the billing is handled on the team level.
To answer this:
Yes, however they will need to be added as a paid editor to the team in order to access and use the shared library.
Hope this helps.