rofl, i did the samething with my heroes/hench going for a 8 man mission to kamadan (max 4), for some reason i had 6 in my party instead of 4 or 8, prob 2 of the henchies not available in kamadan.
It's not an exploit, it's a bug - and quite an old one. This has been happening since before Factions was even released... Imagine seeing a party of 128 players in Fort Ranik.