Tomelyr, we didn't write that function, so you'd likely get best support from the instructions and developer of that script.
Though I didn't study everything, it seems you have to configure group IDs just right, AND, just from the description of the default config method, seems as though one would cancel other out as both are currently true.
--Should it sync according to SMF/MyBB's groups?
local FORUM_to_ULX = true
--Should it sync according to ULX groups?
local ULX_to_FORUM = true