Found some ppl in 'ancient cryptosleep caskets'.

[Andrius227]

Can i make them join me or do i have to arrest them ? I get a warning that their faction will become hostile to me if i do that, but, what is their faction anyway ??? Surely they don't have any friends that could come and attack me, right ?

[Doom]

As far as I know, you'll have to arrest them in order for them to join you.
Also their faction won't attack you, since it isn't an actual faction on your world.

[schwarzrotes]

the faction is SPACER all ppl that crashes with pod are spacer

[Soziele]

Their faction won't attack you, since the other members of it are either dead or also in cryptosleep. You'll have to capture them to have them join you. The pod people usually spawn with guns, so to minimize damage (and have maximum chance to capture them) have someone beat them down with their fists. They can't shoot while in melee, and unarmed damage usually doesn't cause fatal injuries.

[SquareDildo]

The SPACER faction is a placeholder faction, it's currently meaningless.
You can arrest them and recruit them. They often have good stats. In my last danger room I found several people with bionics already implanted! The one I managed to get down without killing had none, but she has something like 8 passions in skills.

[Steelwind]

Also, don't break down the cryptosleep chambers. They make excellent storage for prisoners or wounded if you don't have the time/resources to deal with them. They do count toward your strength or whatever though so I wouldn't go filling them all up for an extended period of time.