The parties were locked back in September so new customers couldn't join. I do not know if the number of people were locked also so if people leave I couldn't tell you if the number drops also as I in party of 18k+ and never paid attention when I left. At some point everyone will have to move to the new plan or leave.
If you were in a party of more than 4 and got the $5 off discount offer you will get it either after you finish the checkout or at the time you activate. I upgraded 5 weeks ago and I don't quite remember what my cart said but once I activated my account changed, the party tab disappears, and my discount shows up as valued member discount.