To add locations to an existing merchant group, first navigate to that group's edit page. Then, click the green Add Locations button.
A pop-up screen will appear where you can select the locations in which you would like to add to the merchant group. Use the dropdown listing under Add Locations to add the locations to the group. Once complete, click the blue Submit button. Your stores will then appear under the Merchant Group listing.
If a location does not show up on the drop-down list, it is because the location has not been added to Mission Control. Click here for instructions on how to add a location to Mission Control.