I usually say "Good morning" or "Which ride are you doing?" or "Hi".
This morning there were 4 recumbents at our group ride, along with 25+ df bikes and one fixed gear. I told the fixed gear rider to go hang out with the recumbent riders because they are weird like him. This got a laugh out of everyone.
Later one of the 'bents was leading the group and he slowed a bit and someone yelled for the "lawn chair" to pick up the speed a bit. Only good natured kidding and they give it right back, too.