Pedgrp.dat is a data text file in the San Andreas data folder; San Andreas/data/pedgrp.dat The file contains the list of ped groups for each group type. The file can be opened & edited with any text editor. To make saved changes(effects), you must restart the game. To comment in the file, put # before what you are going to write.
Again is very simple, for example:
Model, next model,
So, for example, changing Grove Street Families:
FAM1, FAM2, FAM3
# Gang 2 (THE FAMILIES - PLAYER GANG - AKA GROVE)
With Ballas gang:
BALLAS1, BALLAS2, BALLAS3
You need to specify a model, obviously, it should have the same name as it is in the gta3.img file
Note; that you must make sure the commas are placed correctly, this is very important!
Also male01 should not be used in any of the ped groups; as the file states.