People who are not part of a group or organization.
It just means they are not in the group, it doesn't say anything good or bad about them.
Examples:
Nonmembers are not allowed to enter the clubhouse.
The nonmembers don't get the special discount.
We have a lot of members, but we also have a lot of nonmembers.
The nonmembers tried to join the club, but they couldn't.
The nonmembers always feel left out.
The nonmembers want to be part of the team, but they can't.