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Finalize headcount early, then figure out actual names later on. This is because vehicles needs to be clarified and booked ASAP.
However, key members can be chosen earlier. These are people who have long-term buildup of knowledge that are considered either irreplaceable, or if they need to commit early to, for example, complete advanced pilot training and practice flying. Includes:
Pilots
Ground station operators
Key management team/person
Possibly, if the 15-person format is chosen, each subteam lead can nominate/select one of their team members to attend.