user rating other users

Hey guys am trying to create a simple rating system wherein user can rate other users.

To rate or vote other user the user must

1) Be an authenticated user

2) Be part of the group.

Here is an example for better understanding.

In my site i have these users who are authenticated, TIM, JIMMY, SAM, TOM, NICKKY, EMMA, PHIL, ALEXA.

TIM has created a group called ABCD [ i am using organic groups and have provided access to authenticated users to create groups ]

SAM, EMMA and PHIL have joined the group.