Your client should have:
1) an NPI for the group
2) an individual NPI for himself
and
3) a tax ID for the group
The 2 psychologists and 2 psychiatrists each have their own individual NPI's.
For billing purposes on the 1500 form the group tax ID goes in box 25. The group name , address, and phone number goes in box 33 and the group NPI goes in box 33a.
The individual NPI of the rendering provider (whether it be your client or one of the psychologists or psychiatrists) goes in box 24J.
Credentialing is a different ballgame all together.