A useful distinction
Voluntary participation includes the possibility of declining. Repeating an appeal after a clear refusal can change a respectful invitation into pressure. A person may also prefer not to explain their circumstances, and the group need not collect that explanation.
Put it into practice
Record only the communication preference needed to honour the response. Agree on how the team will avoid repeated contact. Keep participation in unrelated community activities separate from whether someone gave money, time or a public endorsement to this project.