"Canada acceding to this confederation, and adjoining in the measures of the United States, shall be admitted into, and entitled to all the advantages of this Union; but no other colony shall be admitted into the same, unless such admission be agreed to by nine States."
I guess the offer's still on! If they do everything states do.
9 of 13 states is 2/3 so it would be 2/3 of 50 today, I would assume.
Yes for other entities, but Canada (which originally most meant Quebec) has a legacy permission to join on the same terms as the original states without 2/3 approval. Technically undone by the Constitution, but kinda sitting back on the table today.