I am always a big proponent of not turning anyone away. I would suggest splitting the interested students into smaller teams with at least one experienced member in each speaking group if possible. Have each group take the lead on a specific outreach event. This will reveal who your best speakers are. At least by giving each person a shot, they all will gain experience with public speaking and outreach. Then as a mentor you should track and keep notes of who is the best, and remember sometimes you will be surprised. Some of my best speakers were the students who started the season as shy and less confident. If you see a glimpse of potential, try to empower them and convey how public speaking can be fun. (involve their families if possible)
Think of the season a year long audition and then when it comes time for competition select the top four or five and focus with them on how to give an effective presentation to the judges. Three will act as your Chairman's speakers and the other two will act as alternates. That way with 5 students, they can take turns maning your pit to speak to judges throughout the competition. (everyone needs a break every now and then, even in FIRST)
As a mentor for my team I tend to challenge the students alot, but it works. I tell them that if they demonstrate effective speaking skills at all times, including in team meetings, it will increase their likelihood of serving as a team presenter (and interestingly enough the Dean's List nominee).
Not that it needs to be said, but our job as mentors is to help every student gain as many skills as possible to help them succeed in their future education and careers. Even if they dont all speak at demos, it doesnt mean it wasnt a valuable exercise for them.
P.S. (one good exercise you can do is to have a family night, where you challenge the students to bring their parents and/or grandparents and explain to them what exactly you do on the team (plus it helps build your team community))
That is alot of suggestions to digest, but I hope it helps.