If one or more characters in my party have hit the xp cap, does a part of the gained xp go to waste?

Ghamarro

Or is it split among the non-capped members of the party?

Grond0

It goes to all of them, whether capped or not (though if you transfer from a save rather than a character export you will find that the XP will include that earned since the last time you saved - so can go above the cap).

jsaving

In my run-throughs, party members usually end up with about 200K XP each though I am only able to make use of 161K XP in-game. I like the idea of letting "non-capped" members get all the XP because that way, you could recruit a missed character late in the game at 32K XP and have some hope they can meaningfully contribute to the party before you reach the final battle. But that's not how it's done in BG/BG2.