I don't think group quotas will do what you want. If you set a 16GB quota on the group, all files that have that POSIX group (regardless of the owner) will count toward the quota. If you want to allow each user 16GB max, you need to give each user a 16GB quota.
If this is for home directories served via AFP, there is a user quota setting you can put in the user record for OD accounts that gets applied when the user logs into his AFP home. You might be able to do the same thing with AD accounts if you extend your AD schema or map the appropriate attribute and set the attribute in the user record.
You could probably script it with cvadmin. Typing help in cvadmin interactive mode shows these helpful commands. Note the "quotas set" command:
quotas [ yes | no ]
Enable or disable quota accounting and enforcement.
Enter quotas with no value to get current setting.
quotas get <user|group> <name>
Get quota parameters for user or group <name>.
quotas set <user|group> <name> <hardlim> <softlim> <timelim>
Set current quota parameters for user or group <name>.
<name> can be the name of a user or group, it can
also be an integer interpreted as a uid (for user)
or gid (for a group). Setting the hardlim, softlim,
and timelim to 0 disables quota enforcement for that
user or group. The values for hardlim and softlim are
expressed in bytes. The value for timelim is expressed
Recalculate the amount of space consumed (the current
size field of the quota record) by all users and
groups in the file system. This command can be run on
an active file system although file updates (writes,
truncates, etc.) will be delayed until quotacheck has
Like quotacheck, but deletes the quota database before
performing the check. Use with extreme caution.