Along with Louise Redknapp - who initially left the group to pursue a solo career and will not be joining them for the show - the chart-topping girl band dominated the charts for much of the 90s with 15 top 20 hits, including Stay and I Wanna Be The Only One.
"Dear All… Day 1 Over," Easther tweeted on Monday. "I was well nervous. Felt like being at school making new friends. Everyone cool though. E x."
Joining Eternal at the rehearsals was R&B group Damage. The band posed for a photo without lead singer Coree Richards - its not know if he will be making a return. But the remaining members, Jade Jomes, Andrez Harriott, Rahsaan J Bromfield and Noel Simpson, seemed to enjoy their first day.
"#DamageDiaries Day 1 at rehearsals and we had a ball!! The show will be electric, the stage will be on fire and YOU WILL LOSE YOUR VOICE singing all these hits in one night!!! #BigReunion."
A1 are also hoping for a comeback on the reality show. Ben Adams, Christian Ingebrigtsen and Mark Read happily posed for a picture together - fourth member Paul Marazzi, left the group in 2002 and will not be returning - later tweeting, "1st day of meeting the Big Reunion people. Gonna be a fun year!"
Girl Thing - Linzi Martin, Anika Bostekaar, Michelle Barber, Nikki Stuart and former Hollyoaks actress Jodi Albert - were all smiles as they posed for their picture, declaring, "The girls are back in town!"
American R&B boyband 3T are also competing in series 2. The trio, who are the sons of Tito Jackson and nephews of the late Michael Jackson, have flown over from the US to take part.
Starting completely from scratch are new group 5th Story. The band has been formed out of former pop favorites Dane Bowers, Kenzie, Kavana, Gareth Gates and Adam Rickett, and the boys have been working hard to get used to their new group status.
"We started out as five strangers but have quickly become a new family," they told fans last week.
"The other bands only have to remember what they used to do… For us it's starting from the beginning. #NewStart."