He won't let you in to see Lares (the bandits' leader) unless you give him a reason: Mordrag sent you / you carry Mordrag's ring for Lares, or you have the Old Mine supply list. Then he grants access.
New Camp, gate to Lares — Chapter 1: Acts as Lares' gatekeeper; you must convince him before he lets you see Lares.
Items Roscoe carries in Gothic 1 Remake.
211
Roscoe can be found in the New Camp (Digger).
You: I have the supply list from the Old Mine.
Roscoe: You ... what? Is that a joke?
You: No. Ian gave it to me.
Roscoe: You, my man, got stones. Let me guess, Thorus sent you to fetch it? He'd absolutely lose it if he knew you were here.
You: Actually, it was Diego.
Roscoe: Ah. Well, HE would be laughing his ass off.
You: So, can I see Lares or not?
Roscoe: Sure. He'll be over the moon.
Roscoe: You again.
You: Mordrag sent me.
Roscoe: Mordrag? Ain't seen that man in ages ... so he didn't turn coat after all?
You: No. He helped me get here.
Roscoe: Uh-huh ... well, if you run into him again, go tell him Lares wants to see him.
You: He actually gave me something for Lares.
Roscoe: What?
You: A ring.
Roscoe: Hmm ... all right, then. Go on in.
You: I want to join your gang.
Roscoe: You and a hundred others, mate. If you want in here, you're gonna have to give me more than that.