I think this worked well in 1999.
All new people provides no continuity.
Negotiate group registration discount from Congress organizer. All U S users who attend the meeting, whether funded by CMHS or not, whether selected by CMHS or not, should be considered part of the official delegation , given any special reception invitations, etc. Otherwise, there is a disincentive to self-funding. Make sure WFMH DC staff know this, and Congress contact. Start the application process early enough so that winners have time to answer the Call and apply to be presenters.