If you accidentally opt-out or need to re-add somebody to a list on request, you can force remove contacts one at a time form the Global Opt-out (GOO) list.

Click the list they were removed from or any list other than the GOO list and then click 'Add Contact' it will ask you to force the re-addition of the number.

This adds the person to that list and removes them from the GOO list. This does not add them back to any other list they may have been removed from though.

Please be careful when uploading large numbers of contacts to the GOO list, there is no way to bulk clear the list.