So far, Friends of the Earth, the World Development Movement, the Campaign for Nuclear Disarmament, and Christian Aid have said they will affiliate.
War on Want says it is yet to decide.
But the Greater London Authority has also targeted less obvious charities such as Help the Aged. "The authority wants money to host the meeting in London," said Help the Aged's international co-ordinator Jo Mihajlovic.
"But we're not sure yet what shape the forum will take. There are a large number of organisations each with different interests, and we don't know yet how it would fit in with our agenda."
The European Social Forum, modelled on the first World Social Forum in Porto Alegre, Brazil, is a mass gathering of activists and NGOs committed to global social justice. The third European Social Forum, after Paris last November and Florence in 2002, will take place on 15-17 October.
The GLA has booked Alexandra Palace as the main venue, but discussions are also taking place about accommodating additional workshops in several venues in Bloomsbury, central London.
Attached to the letter sent out to charities was a form detailing affiliation rates. While funding details are unclear because no financial report has been made public, the forum is understood to be funded largely by the Socialist Workers Party and the GLA. However, several large trade unions, including rail workers union RMT and Unison, are also major contributors.
Tickets are likely to be higher than at previous forums - at least £30.
According to Hannah Griffiths, corporates campaigner at Friends of the Earth, three key themes are emerging under the overarching banner 'Another world is possible' - war, racism, and Europe.
"Opposition to war and racism has really galvanised the movement over the past couple of years," she said.
The forum has been blighted by online reports of in-fighting among the organisers, and accusations that the GLA was trying to hijack the event for political capital. But observers say that many of these issues were resolved at a meeting of the forum's European Assembly in March.
Griffiths said the London meeting was proving invaluable for enabling different groups to network ahead of the G8 meeting in Scotland next year.
"We are really getting to know each other through this process and building a really strong movement."