"hey everyone...we have our 2 (or 4) teams!!!"
follow up question: "who are they?!?!"
Big12: "umm...we'll let them come out and say when they're ready!"
once terms are agreed upon the conf makes a formal invite to the school's board who then votes on it and obviously approves it. the SCHOOL then comes out and says "we've formally accepted an invite to the Big12".