Hi hedingsi,
That 'registration' feature is why I'm looking most closely at Attend Events over at
developer.joomla.org/sf/projects/attendevents
The way Attend Events appears to work is that it relates the id of an event table with 'session' data relevant to registering to an event (like attendance limits, registration publishing dates, and the event's status, among others), then allows users to submit an RSVP to a particular session. So a single recurring event could have many sessions, and one session could have many users signed up (registrations).
There's more planned for it on the roadmap, and it's still being developed, but what's most important about this component is that it's set up in a way that it has the potential to pluck event rows from any component with an event table: Events, Extcal, GigCalendar, etc. to create sessions and registrations. CB integration is in the mix as well, though it's not too clear to me from their configuration options how that would work or could be presented in a plugin.