Thinking about various roles within the team:
Do Strategy/Scout students need constant access to the workshop?
Do Programmers/Control students need constant access to the workshop?
Do Awards/Media students need constant access to the workshop?
Do CAD students need constant access to the workshop?
Are there physical jobs that could be done outside the workshop? For example, pneumatic plumbing, control board layout & wiring, sensor testing & calibration?
At competition, we have a 6-team member max in the pit. Those six people constantly fluctuate, but it's a hard limit. (It also keeps those pesky adults away
) Perhaps you could institute a similar limitation in your shop.
Maybe create a schedule - fab team has access in the beginning of the season; assembly team toward the middle; controls intermittently throughout etc.
But be sure that students who are not in the shop have valuable and structured projects on which they may work. FRC is becoming more and more not about the robot; there are always jobs to be done that don't need to be done in the workshop. And also make sure all students have adult supervision and guidance.