The 27-year-old has just undergone a successful operation, her friend said.
The gathering will take place outside Kings College Hospital in Camberwell, south London, at 3pm on Monday.
Ms Johnson, 27, remains in a critical condition in hospital after she was shot in the head in Peckham, south-east London, in the early hours of Sunday.
The mother-of-two has just undergone a successful operation and her parents are by her side.
Police said there is no evidence that the 27-year-old campaigner was the intended target of the shooting — although the Taking The Initiative Party, of which she is a member, said the attack came after “numerous death threats as a result of her activism” in an Instagram post.
Asked about the circumstances surrounding the shooting, Ms Johnson’s friend Imarn Ayton told BBC News: “As far as we are aware, she was at a party.
“There was a rival gang that may have heard about someone being in that party that they didn’t feel quite comfortable with or trusted and so they resorted to driving past and shooting into the garden, and one of those shots obviously hit Sasha Johnson.
“But I don’t believe she was the intended victim.”
Oxford Brooks graduate Ms Johnson is a known figure in the Black Lives Matter movement and is described as “a strong, powerful voice for our people and our community” by the Taking The Initiative Party (TTIP).
In a statement, the party said: “Sasha has always been actively fighting for black people and the injustices that surround the black community, as well as being both a member of BLM and a member of Taking The Initiative Party’s executive leadership committee.”
The Metropolitan Police issued an appeal for witnesses to the shooting of a 27-year-old woman but refused to confirm if it was Ms Johnson.
She was found after officers were called to reports of gunshots in Consort Road, Peckham, shortly before 3am.