It's really hard to draw the line between inclusion and team performance. Including more people is great, but can cause issues for the team that ultimately degrades the experience. How you handle it goes a long way.
The first point you need to keep in mind is setting expectations. A team handbook can help a lot with this - you can list expected hours for travel and lettering, expected behavior and performance whip in the shop and at competition. Create a process for selecting leadership each year that rewards dedication and leadership qualities, and ensures that the leaders will be around for the meetings (fun fact: of a scheduled 99 build hours this year, my teams two co-captain's were at 104% and 117% - in other words, all the scheduled meetings plus extras we had at the last minute!)
A large part of FIRST is about changing the culture. Unfortunately, science or math clubs are seen in our culture as something that you can participate in occasionally with no big commitment required. Changing the culture puts a FIRST team on the same level as other sports. The time commitment, intensity, and fun all compare with sports. It's hard to create that culture. Many teams have to deal with conflicting priorities, as their students are over-achievers involved in multiple after school activities. Create a schedule that works for your team! While many teams try to meet right after school (after all, the kids are already there, right?), If this conflicts with a significant portion of the team, then find a different time. We meet from 7-9 in the evenings. This allows kids involved in other activities time to do them, eat dinner, and get a little homework done before robotics. It's a win-win!
It won't happen overnight, but it's something a team needs to constantly work at to change. My team had 27 students this year. 19 were present at 50% or more of the meeting hours, making them eligible to travel with the team (a majority of those 8 who weren't eligible to travel were first year members). 15 had enough hours (80%) to letter.
FIRST teams should be inclusive - it doesn't matter if a student shows up 5% of the time or 100%, so long as they are inspired and get something from it. But you need to face reality and utilize everyone appropriately.
For the crowded pit problem, my team uses a schedule. Everyone gets scheduled time in the pit, either as part of the pit crew or doing "pit scouting", and everyone spends time in the stands. Some people may be scheduled in the pit more, base on their expertise, but that is an incentive to everyone to work a little harder
. It's all about having a scheduled job for everyone, at all times, to help the team succeed. If you don't have something for people to do, they'll loiter wherever they want. Come up with a rule in your handbook that gives you a maximum number of people in your teams pit - you need elbow room to work, and over crowding in an area with power tools is a safety issue. Everyone else needs to be outside the teams pit, either in the stands or talking to other teams.