Is it the same group of people that have always done? I remember seeing a bloopers tape of a Seasame Street show, or maybe it was from the Muppet Show, anyway, they stayed in character even after the scene was cut, and they would be swearing at each other in muppet voices. So funny.
Well, most of the originals are still doing the voice, but Jim Henson (Kermit, Ernie, and others) passed away a number of years ago, so someone else has taken over those voices. I’m not sure, but it might be one of his sons.
Offhand, I’m not sure who replaced Jim Henson as the voice of Kermit.
Frank Oz has always been and is still the voice of Miss Piggy, Animal, and others.
According to
http://vr.ncsa.uiuc.edu/BS/Muppets//JHP/bh.html
Brian Henson is indeed very involved in the Muppets, but according to:
http://www-cs-students.stanford.edu/~csilvers/muppet-faq.html#sec15
Steve Whitmire now plays Kermit. Silly me, I was thinking it was Brian all this time.
Here’s a good site with info on the performers. It hasn’t been updated recently, but there is a lot of historical stuff.
According to the website JeffB posted, the voice of Scooter was Richard Hunt. He also did the voices for
Beaker and Janice/Janis (after first season).
Hunt died in 1992. It appears that the character Scooter has been retired. I’m not sure if Janice (the blonde band member) is still around. The voice of Beaker has been taken over by Steve Whitmire, the same guy who now does Kermit.