See the FAQ.
The text document is NuclearThroneTogether.txt. It's one of the 5 files present in every mod release.
/ultras [mode name] Changes ultra mutation behaviour. Supported options are: coop: Normal NT behaviour - coop ultra mutations. own: Per-character ultra mutations (picked for each character individually). all: Both; if a coop option is picked by first player, that overrides both choices.