I assume Art Briles made the comment. Briles is the coach of Baylor (I think). As such, he does would not ordinarily be assumed to have authority to act on behalf of the B12 conference. However, if past circumstances have indicated that Briles does, in fact, speak on behalf the B12, BYU can accept and enforce the contract under the theory of apparent authority. This is not, however, the best course, in my opinion.

The second way to enforce an agreement is to demonstrate that BYU acted in reliance on the tweet, that the B12 was aware that BYU was acting in reliance of the tweet, and then did nothing. By doing nothing, the B12 would be deemed to have ratified the offer and the contract. The way to set this up would be for BYU to send out a letter to the WCC withdrawing from the conference based on the offer contained in the tweet with a CC of the letter to the B12 offices. We give the WCC 24 hours notice of our withdrawal, which isn't enough time for the B12 officials to figure out what is going on and charter a private jet to Provo. By the time they figure out what is going on we'll have acted in reliance on the tweet, and the B12 will have failed to act. Magic, presto, chango: BYU is now in the B12.