Group Join API in Roblox Lua?

In theory, it sounds like it's possible. You click say: A Gui and then Boom! It prompts you to join a clan. or a community, or whatever, But I've yet to find any API for it. So that begs the question, is there one? And where can I find it? Once I know where to look I'm usually all set

So something like:

game:GetService("MarketplaceService"):PromptPurchase(plr, id)

get's turned into

game:GetService("GroupService"):PromptGroupJoin(plr, groupId)

No, there is no such API as of now. It would likely be very problematic if implemented, but might come one day.

Trivia:

"begs the question" actually doesn't mean "raises the question". It actually refers to the logical fallacy of circular-reasoning.