I was accepted to the MSTP off WL on 5/5. Yale sends out their first wave of decisions to everyone at the same time in Feb, but not sure if they do the same for WL offers in May. The Facebook group had 15 members just join — not sure if this is all MSTP or some MD too.
For the group, I also wouldn't worry about how many members there are. It's inflated. On top of people who have already withdrawn, this group also has many current students.
Good luck with WL movement everyone! Feel free to PM me for more specifics, hope this helps!