Determining Group Membership, ypcat etc

My employer has a large network of old RH 7.3 machines. Many users
belong to multiple groups. I'm looking for ways to find all people
who belong to group Robo regardless of if it is there primary or
secondary group memembership. When I use ypcat or ypmatch, I see
those whose secondary group membership is Robo but not the primary
group. Other than doing a "groups"command on every user, what is the
easy way to see a list of everyone who has group membership in Robo
and not all the other groups?