The next evening, at dusk, you all set out.
Thirwin has taken care of his own shapeshifting.
Gregor and Vorel are polymorphed into lizardfolk (and provided with cloaks so they won't stick out quite so much just from equipment and charisma)
Epsilon is polymorphed, also, though he doesn't bother with a cloak.
And Skirrik declines to be shapeshifted at all, preferring to simply never get seen by anyone.
Sneaking this many people into the camp proves rather trickier than Thirwin on his own, even with the help of invisibility. Still, it can be accomplished without arousing undue suspicion - the chaos tribe built their defenses to work against a direct, large scale assault, not a small band of invisible lizards.
Once within the camp, it's not too hard to dodge slaves long enough to find a private spot to become visible again.
Thirwin's "insider" is Sslezza*.
"Ah, you did come... I was beginning to get worried."
"I trust your friends understand that open assistance will hurt more than it helps?"
* character description coming later - for now, suffice to say that she looks like about strength 10, charisma 29 - a stat combination that, in this sort of a might makes right society, means she's probably considered to be one of the rewards you get for being the tribe's leader.