As always, create a backup of the files you modify first

I found this guide to increase the party size in the old spiderweb forums
link[spiderwebforums.ipbhost.com]

Essentially it says, go to the game scripts directory, for me it looks like this

C:\Steam\steamapps\common\Avadon 3\Avadon 3 Files\Scripts

Look for the file z0nightshade.txt, find

change_level(-1,-1,-1,2,-1);

and replace with

change_level(-1,-1,-1,x,-1);

Where x is the number of party members. Use "5" to get a full party.

Now this only works on fort nightshade. When you move to Green Refuge from Fort Nightshade, this reverts to 2 companions. To fix that, just do the same substitution on the file

z5greenref.txt