At 15 weeks these are new experiences she is having and she was growling because she was frightened...To bad you didn't have the treats so the couple (if they wanted) could have helped her to over come her fears. When this happens just move on and refocus her attention. I do not believe you should force a fear issue so if she doesn't want to take a treat from the couple then you move her on to new adventures. Sometimes it does help to pick her up so she feels more secure and then let her become more comfortable with the situation.
Good Luck
