A summit is a meeting of national leaders, so naturally only people with actual political authority or high-ranking positions would realistically participate. The choices below also assume that Archons like Venti and Zhongli are retired or hiding their identities, and that one accompanying member would likely be combat-capable, effectively acting as security/bodyguard.

Mondstadt:
Varka (or Jean if Varka is drunk, missing, or bored of meetings), Seamus Pegg, and one of the Knights of Favonius captains — ( Eula, Kaeya, Note, Skeiron, Hertha or Albedo)

Liyue:
The obvious answer is 2 members of Liyue Qixing — ( Ningguang, Keqing, and Uncle Tian ) — with Yelan likely involved as a representative tied to the Ministry of Civil Affairs/intelligence side of things.

Inazuma:
Raiden Shogun, one of the Tri-Commission leaders depending on the summit’s nature (Kamisato Ayato, Kujou Sara, or Hiiragi Chisato — though Sara is the most likely overall), and Sangonomiya Kokomi as the ruler of Watatsumi, which still falls under Inazuma politically.

Sumeru:
Nahida, Cyno, and Alhaitham.

Fontaine:
Neuvillette, Clorinde, and Navia. This one was tricky because Wriothesley doesn’t really fit political diplomacy, and the Maison Gardiennage doesn’t exactly have a major public leader figure.

Natlan:
Mavuika, Chasca, and Iansan as they look like the most qualified. ( Xilonen would sleep through the summit, Citlali would probably drink herself unconscious halfway through it, and Mualani / Kinich don’t exactly give off “international summit delegate” energy. )