To associate the customer groups to your various shops, go to Clients menu > Customer groups
This interface is dedicated to the customer groups present in your PrestaShop store.
To associate these groups to your shops, display the MutliStore Sharing Manager panel in the Properties column, on the right handside.
Tick or untick the corresponding boxes to associatd or dissociate.
You can of course work in bulk, meaning that you can select multiple groups before associating/dissociating.