I find showjumping friendly because I know so many people now but I think to someone who was on there own all the time it wouldn't be seen as friendly and is quite cliquey.
Having started going eventing this year I have found it very friendly and at one event I forgot tape for the medical armband and asked a lovely young girl and she ran off and got some for me. I find it less cliquey as obviously times dictate when you are there so may miss your friends etc.
Never done enough dressage to really find out and what I have done has been unaff. TBH i find it utterly boring at dressage shows as we are only doing prelim/novice its like watching paint dry!!