STEP-1
Go to, https://admin.google.com > Admin Roles
STEP-2
Click Create new Role.
STEP-3
Define Role name and description.
STEP-4
Choose API permission scopes,
Groups > Read
Users > Read
STEP-5
Verify chosen roles and Click create role.
STEP-6
Click Assign Role > Choose google service user.
STEP-7
Confirm roles assigned and privileges.
STEP-8
Go to API explorer, and Execute.
https://developers.google.com/admin-sdk/directory/v1/reference/groups/get
Enter your Google group mail address > Execute.
Click Allow.