The Prague summit of the European Political Community (EPC) will bring together the European Union's 27 leaders and 17 leaders from the continent currently outside the club including Britain, Turkey, Norway and Ukraine.
Truss's decision to attend Thursday's meeting gives Britain an opportunity to shape a new European forum from the inside after Brexit.
"The British took a very pragmatic tactic - if you cannot 'kill' the strange initiative, it is better to engage deeply and to direct it in your own way," a European diplomat told Reuters.
Diplomats say the EPC's purpose is far from clear and many doubt it will last, with so many foes and friends around the table - even though it will address issues that preoccupy them all including energy, security and immigration.
Macron wants to embrace EU candidate countries losing patience waiting for membership of the bloc and in doing so, counter attempts by Russia and China to gain influence in the continent's southern and eastern fringes.
"One of the points is to be able to tell Kosovo and Albania we can do things with them and they don't have to rely only on Russia and China for investment," a French official said.
Macron worries that the long road to meeting EU membership criteria could discourage Western Balkans nations, boosting populists and Euroscepticism.
The French also think it is important to have a forum to discuss security with Britain, Europe's other big military power, or energy with Norway, which is helping Europe wean itself off Russian gas.
However, Macron's lofty ideals about a happy "European family" are not widely shared.
His proposal was initially seen with suspicion by eastern European countries, and Ukraine in particular, who suspected it was a ploy by France - long reluctant to admit more countries into the EU - to keep them in a sort of "purgatory".
French officials deny this is the case and have gone out of their way to reassure them.
"At the start we were afraid the EPC would be an alternative to EU membership, but as it develops now, I don't think so," an eastern European diplomat said.
However, expectations remain low.
"There are too many countries with too many interests ... How do you have Serbia and talk about Russia?" the eastern European diplomat said.
"How do you have Turkey and Greece/Cyprus together? How to have Armenia and Azerbaijan at the same table?"
While France is aware many feel the summit will amount to little more than a family photo of leaders at Prague Castle, it is pleased Ukraine has come up with proposals on what the EPC should look like and Moldova has offered to host the second summit.
European diplomats say the initiative will also bring Turkish President Tayyip Erdogan closer to Europe and away from Russia.
Turkey opened membership negotiations with the EU 23 years ago.
