Ok. So the PCs range from levels 6-8 (6-11+ if we include the wizard) and new characters start at 6th level. (i.e., they start at the same level as the lowest character in the party, maybe?)
Either way, I just don't fall into the "let's give someone XP even if they weren't here" group - I'm cool with starting PCs at a higher level, but not giving absent players XP.
See, that's exactly the problem here, though.
Having new characters come in at 6th level is all fine and dandy in a party whose levels range from 6-8. So is not giving xp to absent players' characters.
Where it all breaks down here, though, is that this particular level 6-8 party also includes a single ca. 11th-level wizard.
Now, the 8th-level characters can still be somewhat useful, assuming they fill a different role in the party than him. But the rest of you are just plain screwed.
Now, I've never given xp to absent players, either. (Kinda defeats the purpose of actually risking your life and limb if you'd get the xp anyway, doesn't it?)
But if you reach a point at which some PCs overshadow the others by so much that the weaker PCs just don't feel useful any more (
and objectively aren't), the DM would do well to intervene to correct the balance a bit. *shrug* Whether one likes to give PCs xp they didn't "earn" in-game, people having is much more important than maintaining any arbitrary pretensions of who "earned" what. And you don't seem to have as much fun as one would hope, so a good DM should take action.
Not that correcting the balance is particularly easy at this point, mind.
Really, if I was the DM in this scenario (well, except that I'd not be female or married, of course

), I'd have a hard time deciding on how to correct the problem.
Hm. Right now, I see two options:
1. Increase all 6th- and 7th-level characters to 8th level and increase the previous 8th-level characters to 9th level. (D&D 3.5 suggests that a 2-level difference isn't a problem and that a 3-level difference isn't a big problem.)
If anyone minds characters getting xp they didn't "earn," they should get a life.

Failing that, I'd insist that all weak characters be retired and replaced with more suitable traveling companions for the great wizard (insert-name-here). These new characters start at, say, 9th, 10th or 11th level.
2. Remove the uber-wizard from the party and let his player bring in a new 8th-level character as compensation. He can still continue on with solo adventures, a new party or whatever but he's too strong to hang with the weaker members of the current party. (And really, given his other advantages, he overshadows the 8th-level characters as well - just not by so much that DM action
needs to be taken.)
In any case, if the same problem is to be avoided in the future, the character(s) of the highest level should not be allowed to go on very many adventures without the rest of the party.
(Disclaimer: Dear reader, if you have a big level difference in your campaign and it works well for your group, more power to you. Note, though, that this post isn't aimed at you; it's for those who
do have a problem of the aforementioned nature. Thank you.)