The strength of the other party is crucial in this situation.
If the opponent is weak, then there's no point in teaming up; it's only a small piece of meat, hardly worth anything. Why team up for that? To starve together?
If the opponent is extremely strong, completely crushing, then it doesn't matter if we team up or bring ten more people along. It won't change the result. It's like a village teaming up to attack a big city. It's not a matter of numbers but of whether it makes sense. Is one person dying not enough, so a whole group needs to go to their deaths?
Only in a situation where both parties are relatively equal is there a need to team up. If two small groups are clashing and each side only has a few people, having one more person or one less person could make teaming up necessary, like making a business deal that's just short by a small amount, forced by circumstances to team up.
Teaming up is only necessary when things are certain; it's not a universal solution.
