I don’t necessarily make a belief that is literally “Stick together team!” But I do always make one belief about the other party members. If you didn’t have some attachment to them, why would you be with them in the first place? It just doesn’t make sense.
It doesn’t even have to be a positive belief. It just has to be some kind of reason that you are with them.
Maybe you are indebted to them, and you always pay your debts because of honor.
Maybe you are very greedy, and they are paying you enough for you to stay with them.
Maybe you are afraid of someone else, and the party provides safety.
Maybe it’s just a convenient arrangement. You’re a nice person, they are nice people, and you’re all going the same way, so why not team up?
You can also always put in a stronger connection. Maybe you are related to each other by blood. Maybe you have been life long friends? Players, self included, tend to always make individual characters, and then find a reason for them to come together. Why not make characters that are very very close to begin with?