How to facilitate meetings / events with participant registration, agenda and documents?
There are many options in Drupal allowing you to add registrations to content. So if you add a content type of 'meeting', you can add file fields, dates and an agenda text field. Add a taxonomy field for grouping by type.

Then with any of the registration modules you can register users. 


Entity Registration
Node registration
RNG (Drupal 8)


Basically it is not one module, but a combination of fields and modules to get the desired functionality.