How can I use access groups to give user A access to the records visible to User B

I currently, have the need to provide 'proxy' access for user A to the records visible/editable of User B. However, I've not been able to setup the rules correctly and am wondering if there is an example to apply.

I need user A to have the same access to the records of User B and visibile to this user.

As of now, I created an Accesss Group which consists of 2 members who represent user A and B. I subsequently added the rule that the access group members should be able to see the, for example, lead records 'owned' by the access group members. Unfortunately, this does not seem to provide any access for User A to the leads owned by User B.