I just mind my own business and treat them as any other motorist out there. I don't think just because we have the same car that I am obligated to go out of my way to wave and stuff, nor be on the lookout for other M3s so we can share a "moment". I've met plenty of people in parking lots that make nice comments about my car or approach me with a question, ///M and non BMW people, and I am always friendly to these folks.
I make it a requirement to wave when I pass other ///M drivers on the track tho
