Why was Boromir allowed to join the Fellowship?

[Inziladun]

Originally Posted by Tuor in Gondolin

Didn't Aragorn say that he had thought to go with Gimli and Sam with Frodo to Mordor, and have Boromir take the other hobbits to Minas Tirith. Not sure who Legolas would supposed to go with.

That was Aragorn's thought for the Company as the situation stood after Gandalf's "death", but I was musing on what they would have done if Gandalf had still been with them.