until now, ${u} would match all users,
${u%-foo} would exclude users in group foo,
${u%+foo} would only include users in group foo
now, the following is also possible:
${u%-foo,%-bar} excludes users in group foo and/or group bar,
${u%+foo,%+bar} only includes users which are in groups foo AND bar,
${g%-foo} skips group foo (includes all others),
${g%-foo,%-bar} skips group foo and/or bar (includes all others)
see ./docs/examples/docker/idp/copyparty.conf ;
https://github.com/9001/copyparty/blob/hovudstraum/docs/examples/docker/idp/copyparty.conf
