lists all users under groups

[Emi Lu <emilu@xxxxxxxxxxxxxxxxx>]

Greetings,

May I ask is there a simple way like \dg+ to get all users' names under 
all groups please? If no, which sys tables should I use to get it?

I am expecting something like :

Group name   | Users
----------------------------------
g1  |      u1, u2, u3
g2  |      u1
g3  |      u6

Thanks a lot!
Emi