To initiate co-op, approach the portal, Chronosis, at the start of a biome (for example there’s a portal in the starting area, where Helios is located). From there, you can choose to host a public cycle, where anyone can join your session, a private cycle, where only someone with an invite will be able to join your session, or join a cycle and enter another player’s session.
If you are the host of a session, you will be able to gain permanent equipment, but you won’t be able to increase your Scout Rank. When ending a session, the host keeps their progress. If you are the joining player, you will be able to increase your Scout Rank, but unable to gain any permanent equipment. A joining player cannot access their permanent equipment which the host has not yet acquired. If you join a game, after the session is done, you will be returned back to the start of your cycle.