changing group membership as a non admin

hi all,

I have a user (user A) who edits group membership using the following method;

1 - opens outlook

2 - clicks new to create a new email

3 - then clicks the "to" button

4 - then right clicks on a user and selects properties

5 - there is then a member of tab

6 - user A can then select which group the selected user is a member of

I need to enable another user in the organisation (user B) to do the above but I cannot find which permissions or group membership user B should have.

Does anyone have any idea?

Thanks