Johannesburg - Mamphela Ramphele held a “private meeting” with AgangSA officials at the party's office in Johannesburg on Thursday.
Security officers guarded the door of the Braamfontein office, and reporters were told Ramphele would not be available for interviews.
AgangSA's official Twitter account posted photos of Ramphele dressed in a green party T-shirt as she addressed party officials.
A tweet accompanying the photo read: “Our members sing “(at)MamphelaR lo, my president” as the leader enters.”
Another tweet quoted Ramphele saying: “I respect and love all of you. This is for the greater good of the country. - (at)MamphelaR as she addresses our members #AgangSA2014.”
On Tuesday, Democratic Alliance leader Helen Zille announced that Ramphele would be the DA's presidential candidate in the general elections this year.
AgangSA reportedly intends electing another leader to contest the general elections. However, the DA has said the two parties planned to merge.
According to AgangSA's tweets on Thursday, Ramphele emphasised that she remained passionate about the party and the dream for a better South Africa.
Some supporters of Ramphele had expressed disappointment at her move to become the DA's presidency candidate.
A twitter page claiming to belong to AgangSA spokesman Thabo Leshilo tweeted that he felt duped by Ramphele's move.
Both Leshilo and AgangSA have said on Twitter that the profile was a fake.
AgangSA tweeted: “Please help us report (at)Real_Leshilo as spam. This account is impersonating (at)leshilot.”
By 2pm on Thursday, the account had been suspended.