Disregarding system scripts like getent, users can be in a group in two ways: either it is their principal group, defined in /etc/passwd, or they have it as a secondary group in /etc/groups.
GROUPID=1000
sed -rn "s/^[^:]*:[^:]*:$GROUPID://p" < /etc/group | tr ',' '\n'
sed -rn "s/^([^:]*):[^:]*:[^:]*:$GROUPID:.*/\\1/p" < /etc/passwd
Should do it nicely. awk would probably look nicer...
GROUPID=1000
awk -F: -v "g=$GROUPID" '{if ($3==g) print $1;}' /etc/passwd
awk -F: -v "g=$GROUPID" '{if ($3==g) print $4;}' /etc/group | tr ',' '\n'