Listing RealNames of all the users of an Open Directory group in MAC OS X
There's probably something wrong with this or an easier way to do the same thing, but could you just run dscl . -read /Search/Users/username RealName for all usernames?

g=admin; dscl . -list /Users | while read l; do printf %s "$l "; dsmemberutil checkmembership -U $l -G $g; done | grep 'is a member' | cut -d' ' -f1 | while read l; do echo "$l $(dscl . -read /Users/$l RealName | tail -n1 | cut -c2-)"; done