No idea yet. What I said above is the starting point. Where we go from there depends on many factors. We'll try it first in the crawler, see how it works there (assuming we can make it work), what changes need to be made, etc.
While typing it, I thought of two solutions:
a) quality vs quantity. For example, CHA6 can get you either 2 average party members or one with great stats and skills (i.e. "I'd join you but you have too many people" response).
b) XP split. The more people you have the lower everyone's skills are. In some games it's not a factor, in AoD it is. So 4 people can be less effective (hit less often, take too much damage) than 2, for example.
How is a) a solution? You still get a huge power increase with more CHA even if you opt to take less but better people with the gains. Otoh if there was just 1 or 2 superb teammates, but only mediocre ones after that, would make the power increase that comes with CHA more diminishing.
b) of course works as evidence by a lot of RPGs, but I find it extremely boring.
Other ways to balance different sizes of teams (whether the teamsize's tied to CHA or not):
Equipment matters more than numbers. If you've got limited number of firearms, extra men will help a p. limited amount, one guy in power armor is worth 5 men without, etc.. Also for non-combat stuff, you've got a two anti-radiation suits that are needed to operate in a given area.
Or logistical consideration in the same vein. The elevator can fit three people (and power armor takes the space of two) and the combat will start right as the doors open. Sneaking gets exponentially harder with more people, even if individual skill was constant.
People cost resources. Like you have to pay your mercs in JA. Could be food or whatever that's scarce there, having a barely useful hang-around in your party is worse than an empty space. Or whatever personal demnds bullshit the people have, the more of there are of them the bigger the hassle. Would also be balanced that the best people don't want to work alongside each other for some reason.
Have some things you really, really want the main PC to do instead of telling a party member to do it (Some things they might just refuse to do, betray you for even asking, or blackmail you afterwards). So that there's a bigger gain from investing in a non-CHA stat.
A bigger party attracts more attention from everyone. This can be a good thing, that people know you have more people behind gives you a position of strength, but might also catch negative attention making you a target, disallowing some subtler methods.
Tying the allowed teamsize to something else than charisma could be more interesting (either it be free for all, or a skill that you can raise, or smth else). It's an interesting idea to me that a you're playing an uncharismatic guy trying to lead a lot of people, but due to the low charisma can't properly control them or make best use of their abilities.
One thing is though, that if your PC is supposed to lead a team in the game, leadership (not the same as charisma, a stand-up comedian is not the same skillset as a CEO or a sergeant) is the most important thing. Specialists in other fields are supposed to be his subordinates.
* I don't know anything about the spaceship game, what will you actually be doing in it.