What We Know
GRAIN ROT uses player-hosted Steam lobbies. The official FAQ says friends join through a Steam invite or the Steam friends list, and that there is no public matchmaking.
Steps or Conditions
Have the host create the Steam lobby, confirm both players are visible to one another in Steam, and send or accept the Steam invite. Each player needs their own copy of the game and should be on the same current version.
What Does Not Work
Do not look for a public queue or rely on unverified NAT, port-forwarding, or cross-platform instructions. Those steps are not supported by the approved evidence set.
Evidence and Limits
This answer uses the official GRAIN ROT FAQ. Steam presence, friends-list visibility, lobby state, and matching game versions are sensible checks; a new official lobby change requires a fresh review.