i arrested someone and when i moved to the ship assigned to me the prisoner was left behind. since i am not part of the crew from the other ship i can't go back to get him.
i was working for him as a pilot of the ship were he is stuck, now i can't even make a party with him

Did you check to see if there was sufficient room for both of you on the ship that you moved to?

I mean, if you tried to move with him to a 1-man fighter, there wouldn't be room for him, just you.

the new ship holds 16 people, there ara only 3 so he should enter.
my problem is how to go back to his ship because he's the only one who can add me to the crew list but because he's been arrested he can't do it anymore and i can't release him or had him to my party because i'm not in the same ship as he is

Nine to one that Joao here didn't add the arrested guy to his own crewlist before trying to drag him across.

Didn't think you needed to add people who were in your party.

You need to add PC in your party, but not NPC in your party. Not sure why the difference.