Strategies for getting involved in students' work during labs?
From your post it appears that the labs are already highly interactive. Students are engaged working on the assignment among each other. From an education perspective the current learning environment is student-centered, which is often the ideal.

However, one common drawback to an engaging, student centered environment is that the teacher is not the focus of the learning. This can be somewhat un-stimulating for the teacher but the goal is always for the students to have maximum involvement. 

One option for dealing with this situation is to shift from trying to lead the students to provide expertise when necessary. When students have questions be available to answering them. Another useful strategy is monitoring. This involves observing student progress and providing additional unsolicited support.