I actually made a list of notable concert attendees a while back. Here's my addition to this thread.
Birmingham 11/27/73 - Rob Halford (Judas Priest)
London 12/14/73 - Gary Numan, Freddie Mercury's parents, David Bowie, Steve Jones (Sex Pistols)
London 3/31/74 - Simon Townshend (brother of Pete Townsend)
London 11/19/74 - Brian May's mother, Gary Numan
New York 2/16/75 (Early) - Ivan Kral (Blondie)
Seattle 4/6/75 - Nikki Sixx (Motley Crue)
New York 2/xx/76 - Elton John
London 9/18/76 - Roger Waters (Pink Floyd)
Montreal 1/26/77 - Ian Hunter (Mott the Hoople), Roy Thomas Baker
New York 2/5/77 - Brian May's parents
Inglewood 3/2/77 - Michael Jackson, ELO, Cozy Powell, The Beach Boys
Seattle 3/13/77 - Heart
London 6/xx/77 - George Michael
Richfield 11/27/77 - Bev Bevan (ELO)
New Orleans 10/31/78 - Robbie Steinhardt (Kansas)
St. Louis 11/23/78 - Heart
London 12/17/79 - George Michael
Inglewood 7/xx/80 - Michael Jackson
New York 9/xx/80 - Bob Marley
London 12/xx/80 - Freddie Mercury's parents and Brian May's parents
London 12/9/80 - Pete and Karen Townshend
Tokyo 2/13/81 - Gary Numan
Buenos Aires 3/8/81 - Diego Maradona
Montreal 7/21/82 - Shawn Drover (Megadeath)
Inglewood 9/15/82 - Michael Jackson
London 9/7/84 - Rick Parfitt (Status Quo), Brian May's family.
London 9/8/84 - Roger Taylor's son
Auckland 4/13/85 - Tony Hadley (Spandau Ballet)
Melbourne 4/16/85 - Phil Collins
Leiden 6/12/86 - Fred Mandel
London 7/12/86 - Mick Jagger