Yes, I have made a similar post before, however that post was about offering my help to new players, not a straight guide. With that out of the way, lets begin!

The NPC crew in Railjack is definitely worth your investment. Especially if you dont want to play with other players, the NPCs can actually pull MORE weight than a player would at some points!

Engineer: Repairs any onboard hazards or faults with his Omni, which for some reason has no ammo system. This means as long as you have an NPC on engineer, the ship can not die unless you get boarded. Oh, he shoots boarders too when not repairing something.

Pilot: Will attempt to drive the ship to the nearest yellow marker, and if none exist, will approach crewships and fighters for combat. Will also line the camera up for artillery shots from players in the appropriate seat. You can take over from Pilot at any point and the NPC will patrol the ship for boarders while he waits for you to leave the station.

Gunner: WIll take control of the side guns and act as 360 degree coverage for fighters, crewships or even ramsleds. Equipping these guys with a hitscan weapon like Talyn turns them into space Mesas. Just like Pilot, you can take their seat and they will wander around while they wait.

Defender: Arguably the least useful role, as everyone else will do this when they have nothing to do at their stations. Defenders are solely on look out for boarding parties. They will ignore hazards and hull breaches, they will ignore everything except boarding parties. In my opinion, defenders are a waste of a crew slot.

ALL crew members CAN: revive downed crewmates, including players, if they dont need to leave the ship to reach them. You can change their role at any point by opening the Tactical menu and choosing a new role for them to fill. They can also all mimic your Integrated Railjack mods, including the aura, and will bring one of your own fully modded weapons with them into combat.

All crew members can NOT: use the Forge, use the Forward Artillery, or leave the Railjack.

My recommended layout for crewmates is an Engineer on slot 1, to make sure they dont leave if a player or two joins, and two Gunners. This makes sure ramsleds dont get in and attack your Engineer! You can also have one of your gunners be a backup Pilot in case you need to do a grineer yellow marker, and want you NPCs to help with the radiators.

Let me know if there is something I missed. Thanks for reading.

My other two guides:

Parts: https://www.reddit.com/r/Warframe/comments/1ajj79f/railjack_component_guide_that_does_not_just_list/

Mods: https://www.reddit.com/r/Warframe/comments/1ak3h6d/railjack_modding_tips_that_will_help_you_even/