They join previously-announced headliner Rita Ora at London's Victoria Park on Bank Holiday Sunday, May 26 from midday to 10pm.
Further additions to the bill include former Frankie Goes to Hollywood frontman Holly Johnson.
Skunk Anansie frontwoman Skin will play a DJ set, and Israeli DJ superstar Offer Nissim also performs.
An additional five stages and four indoor arenas will be announced in the coming weeks, and more live acts and DJs will be announced shortly.
The Circuit Arena will be presented by Matinee, WE Party and The Week, and the Best of British Arena will be hosted by Beyond, Orange and Salvation.
Other brands involved in the event will be Delooded, Shadow Lounge, Bearracuda, Madame JoJo's, Findhrr, Girl Action, L Fest, Mint, Code, Ku Bar, Rupert Street and Comptons.
Earlybird tickets are priced at £35.
VIP passes are priced at £75 and include fast-track entry, a glass of champagne, access to the Shadow Lounge in the VIP Village behind the main stage, a VIP bar, table service and VIP toilets.
Watch Union J talk about their time on The X Factor below: