I'm using Affinity Designer 1.4.2 and I could not figure out how to edit a group. I double clicked the group to no avail. Finally I realized that I actually could edit the group, but there was no visual cue to tell me that I was in edit mode. I think having a greyed out transparent background would be helpful (similar to Adobe Illustrator's isolation mode). This would tell me that I'm now able to edit the group.