Elrond never sent out a summons, but the attendees of the Council, like Gimli, Legolas, and Boromir, happened to have been in Rivendell for reasons of their own. Boromir came to Rivendell for interpretation of a disturbing dream he and his brother had, Legolas came to discuss the escape of the creature Gollum from the care of his people, and Gimli was there because Mordor had sent an emissary to his people asking after Bilbo Baggins and his Ring.