You must mean the ABBA tribute Band that performed at the American Pavilion Amphitheater during the "Sounds Like Summer" concert series this year.
The group was called "Bjorn Again" and they were there July 22-Aug 6. They were fantastic- not only did they sound like ABBA, they even looked like them.
Here's a link to their site:
http://www.bjornagain.com/
Also in the series were tribute bands- Transit Authority, really amazing tribute to "Chicago"; Hotel California- tribute to the "Eagles"; Draw the Line- Aerosmith clones and 52nd Street- For the Billy Joel fans. The Transit Authority was excellenet- dare I say it?- better than the current Chicago line-up. We went over several nights during the two weeks (first in June, then they came back in August) they were there. Now I'm a fan!
I sure hope Disney does this again in 2007, it was really entertaining, interesting and great music!