Chapter Serf
Space Marines are big tough supermen and all, but even they need a helping hand. Someone has to to polish their armor and nob, cook their food, sweep their halls, clean their toilets, file their tax returns, and keep the fortress-monastery defended when the brothers are out stomping Bloodletter faces into the ground and tearing Ork rectums apart with chainswords. These people are Chapter Serfs and live comfortable, well-to-do lives... at least by Imperium of Man standards.
Chapter Serfs (formerly Legion Serfs), were originally picked from failed Space Marine Aspirants and other natives of Space Marine homeworlds who, though physically unable to become Space Marines, still wished to serve. The children of Chapter Serfs also often become chapter serfs themselves. Generally speaking, Chapter Serfs are well-trained, highly educated, and treated well; they are officially part of the Chapter. They also have access to highly-sophisticated weaponry (even the Imperial Guard don't get access to this shit). Of course, that does not include Marine Bolters. Not out of elitist exclusivity or anything, but rather because they are heavy and far too unwieldy for a regular human to use to any degree of real effect as a weapon.
How well they're treated depends on the Chapter they serve. Ultramarines, Blood Angels, Space Wolves, and Salamanders treat them as equals to other Space Marines, like soldiers might treat their friends in the supply corps. Iron Hands and Imperial Fists and their countless successor chapters see them as just kind of there and don't really care one way or another, the way you treat the fast food workers and garbage people who make your comfy life possible. Marines Malevolent, being dicks, treat them like slaves. Flesh Tearers keep them as something to murder between missions. Blood Drinkers kill them and drink their blood to stave off the Red Thirst. Don't even get us started on how the Legion Serfs in the Traitor Legions are treated (like really bad, imagine being forced to watch erotica berzerker porn with bloody panties and tampons. Don't even think about it! Some are treated well. Talos of the Night Lords treats his serfs very well.) More often though, Chaos Warbands doesn't have actual Serfs, but rather use Cultists, who are pulled towards the Warband to worship them. They are very often treated like absolute shit by the Traitors, since their own hubris and newfound freedom doesn't go well with 'being nice to people below you'. If nothing else, most Chaos Lords knows how important a healthy supply of fanatic cults is for the runnings of a Warband.
In summary they are the Alfred of 40k, because no fine super hero are without a friend/allies that would comfort them after all the grimdark shit happened outside to fight another day.
Rules for Serfs
Chapter Serfs are represented with various rules both in the 40kRPGs and on the tabletop:
On the tabletop (both in 40k by the Badab War books, and in 30k Extermination) they are represented in battles where space marine capital ships get invaded by enemy forces, where the crew has to get involved by necessity. They come in squads of 6, are armed with either laspistol & CCW or Shotguns and wear void-hardened flak armour. However they have the same stat-lines as Imperial Guard Conscripts because they aren't battle-hardened fighters, or expected to be.
The Deathwatch (RPG) does it very similarly, having stat lines weaker than starting Dark Heresy player characters, and if you take the tens digit of their statline it very nearly exactly follows the tabletop stats for them too. What is different about them though is that chapter serfs do have biological enhancements, and have the Unnatural Strength, Toughness and Senses traits, as well as Dark Sight. Which actually falls into line with how the main character of Prospero Burns is presented. (though he's not strictly a Serf, his enhancements were apparently no great expense for the Legion)
However, with the ally rules, you could probably take them as Imperial Guardsmen, just modeled differently. What to do with all those Spare Marine and scout torsos? Give em lasguns and play them as Chapter Serfs