If I'm trying to level my lvl 95 character in a Chaos/Baal game with other characters that are level 99, do they still gain experience? Would I get more if they are already at 99?

No

You get less exp.

They take the exp like they are leveling but it just does nothing for their char.

The way exp is calculated in party it gets pro rated to the level

So a level 90 in a group of 80s gets more exp than the 80s (before experience penalty is factored in)

well, that's a bit disapointing. thanks for the info.

Try kill nith as well