New party members start at a level based on the average of all your existing characters (although I think it is possible that some characters start at your average level plus/minus 1 or 2). There is a point in the game where you temporarily lose access to everyone but a single party member. When you reacquire the other party members after that event, their levels will now all be based on the level of this single character you were in control of.
The first character you rejoin will be of the average level of that single character, which is of course the level (but again, there could be static boosts/penalties for some characters). The second character you meet will be of the average level of the now two party members you have, and so on. Until you have reacquired more characters than you can fit in your party, they will all start at roughly the same level.
This event is pretty far into the game, and you'll "probably" know which one it is when you get there. Until after this point, you never really need to worry about leaving some characters behind, level wise. You can't permanently screw anything up no matter which characters you choose to use, so don't worry too much about it.
Losing scenes because you brought the "wrong" characters is just a good excuse to play the game more than once. Replay value, you know!
edit:
After gaining over a dozen characters, it is actually possible to choose to not re-recruit them again after they temporarily leave. You can technically beat the last boss without having a character pool of more than 4 characters. It is not recommended for beginners though, seeing as those four still need to split into three different parties where they'd fight either alone or as a duo.
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