You can only use reference selection from the entity owning the relation.
In your case
Tasks can have a reference selector for Meeting
Attendees can have a reference selector for Meeting.
I'm not totally sure what you actually want to model, but the straightforward solution seems to be: use a reference set from Meeting to Attendees, then you can use a reference set selector.
Edit:
https://docs.mendix.com/refguide/reference-selector#constrained-by