Different releases of the program had different guests. In the Scandinavian version, Agnetha Fältskog and Anni-Frid Lyngstad introduced Ted Gärdestad who sang "Take Me Back To Hollywood". In the Swiss version, Björn Ulvaeus introduced figure skater Denise Biellman who skated to "Money, Money, Money".
Released in 1979, this show in effect served as a promotional tool for the tail end of their music catalogue in the late 1970s, when they were still active, but hadn't officially separated as a group.
Clearly, if all the "supporting" acts, only shown in the localised versions, were edited into the main ABBA, in a festival style running order, this show would easily be a 2 hour TV special, in aid of UNICEF in 1979.
This is the TV special where they perform in a vast marquee surrounded by an audience on three sides of the stage, and do chat pieces straight to camera, often in snow.
In the version the UK's BBC Two re-ran in November 2021, as part of their ABBA themed music night, the musical guests were Roxy Music and and Kate Bush. It appears that ABBA, Roxy Music and Kate Bush were all actually lip-syncing to their original studio recordings, rather than lip-syncing to re-recordings for the TV special, or even performing live vocals and or instruments to backing tracks.