there was an episode of Torchwood (Doctor Who spin-off, Welsh team fights aliens or summat) where a bloke just turns up and does some mind control shenanigans, as if he's always been part of the group.

i don't have any interesting stories to share, but you could totally just get away with infiltrating some social circle by pretending you fit in. e.g. have wandered into an anime convention for free and (i think) they assumed i was staff. just walked in.