Anthony Costa?

I don't know about the others, but Una released a solo EP before the group formed.

Frankie also did a song with another group about a year or so a go.. not exactly a solo career but it was a song where she got a solo credit.

John Taylor from Duran Duran had a solo with 'I Do What I Do'.

Also from Duran Duran,Andy Taylor released some solo material in the late '80s,including the 1987 song 'I Might Lie' which Armand Van Helden used as the main sample in his 2005 minor UK hit 'Into Your Eyes'.

Jason Orange from Take That didn't.

Certainly not ABBA. Agnetha and Frida both had solo careers before ABBA, and then again after (Agentha even had three solo albums during the ABBA years).

Agnetha had her first number one album in Sweden at 16 years old and Frida was the lead singer in a jazz band, touring Sweden, when she was just 13 years old.

Benny and Bjorn were both with different bands before ABBA. Benny was with 'The Hep Stars', Sweden's most successful rock 'n' roll group of the 60's, and Bjorn was with 'The Hootenanny Singers', a Swedish folk group. Since ABBA finished, Benny has worked with his new band BAO.

ABBA were something of a supergroup when they formed, as all four members had successful careers in music prior to the band. Benny and Bjorn initially had no interest in forming a band with their girlfriends, and just wanted to work as writers/producers. B&B performed over 200 concerts each with their respective pre-ABBA bands and were already tired of performing.