In the "File inspector", "Identity and Type", simply change the location to "Relative to Group" or "Relative to Project", the path shown beside the little folder icon is automatically updated. There is nothing more to do.

"Relative to Group" is the default choice. By the past there could be some strange issues related to the fact that a group didn't necessary have an equivalent folder on disk. Nowaday when you create a group, a folder is automatically created too, so it becomes difficult to have this kind of problem. So I don't see any reason not to use the default behavior.