You decide who you wish to add to or remove from the grouping list. It is easy to do, once you know who.
UFTree paper manual p. 128-129.
Personally, I never create an individual record for the minister, official, or even an unrelated housemaid. I put stuff like, 'Marriage performed by Justice of the Peace John Smith' in as text to the marriage event or as text into a biographical text box. Of course, if John Smith is 2nd cousin to the bride, that is different.