The project manager should discuss the amount of work and timeframe required for the project with potential members to make sure they can fully commit to the project. That timeframe should include vacation days, and additional days for unexpected emergencies. After the team is formed, members will need to log in their expected absences on a communal calendar at least a week in advance. Around five to seven extra days should cover emergencies for all of the team members. Also, if a member uses up too many emergency absences, the PM may need to cut into or delay their vacation time to keep everything on schedule.
If possible, the PM can hire one or two extra part-timers who will be kept up to date by attending design review meetings. They can then fill in whenever too many full-time members are absent.