How to change default approval rank?

Hello Community,

My registration configuration is set so that new users that register have to be approved by an Administrator first. Everytime I approve a new member he/she gets their group automatically changed from Guest to Member. I however, have content on my forum that shouldn't directly be available for them. So everytime after I approved them I also have to edit their profile and assign them a new group that has less permissions than a regular member. I call this group 'Registered Guest'.

Is there a way to change the group the button assigns the user when I hit "Approve"?

Thanks in advance