According to their twitter accounts (see below) both are flying to Arizona today (Monday).
@m_agosta17 - Off to AZ tomorrow! #journeybegins
@mwilliamson7 – Off to Arizona! I will be a Giant soon! Thanks to every single person that has helped me get where I am today!
If you go to Agosta's twitter page, you'll see that he's already updated his profile:
Pitcher in SF Giants organization
If they follow past tendencies, both guys should be getting their medical exams today, and the Giants will likley be announcing both signings on Tuesday. I also think that we'll likely to be seeing Stratton, Agosta and Williamson making an appearance at AT&T Park and being introduced to the fans at some point later this month. They'll probably pay to have their parents flown out and give them the full-court press and red carpet treatment.
Agosta pitched a lot of innnings as a starter for St. Mary's this spring, so I don't think we should expect much more than some bullpen work from him this summer. I would guess that he ends up in Salem-Keizer with Chris Stratton, but he could also stay in Scottsdale if they want him in a more controlled environment. The Giants will likely keep him from making his pro debut for at least 3 weeks as they slowly work with him to get his arm into game shape again.
Williamson should be ready for his pro debut much quicker. I think he'll be on the field again in about 10 days - most likely with Salem-Keizer.