I was also wondering where were the rest of the crew. Maybe Riker chucked an early Kirk and insisted I'll deal with these intruders myself as they wronged me personally. Even if thats the case I don't think its Riker's style to endanger the ship like that.
Or maybe the rest of the crew were looking somewhere else. Was it explained why they just couldn't transport the intruders out?
NEMESIS didn't mention the intruders outside of their existance and Picard's order to Riker to go handle it. This movie doesn't go on to explain much of anything it introduces, which is some of the laziest writing I've ever encountered in a movie.
Shinzon doesn't explain the source of Picard's DNA he came from. YES, I know that Shinzon states that the Romulans "somehow" got a hold of it, inferring that he doesn't know. But that's more John Logan's fault than Shinzon's, let's face it. Also, B-4's origins are not explained. In fact, "we" can't even be certain that B-4 and Data were even created by the same man. Everything stated about B-4's likely beginings is pure speculation, on the part of the crew. What's more, Shinzon does not explain how he found a replica of his own replica's favourite android. Nor does Picard probe Shinzon further on this, at all. This script steadfastly refuses to explain anything about itself ...