If you were a new customer, you would have to wait one week, but you have already cleared a charge so you should be able to exceed the 100 user quota immediately. You may want to try to send mail and see if you get quota exceeded exceptions.
The "Leave Grace Period" messages mean that a charge previously failed putting you in the Grace Period. You then later successfully charged your credit card and left the Grace Period.
App Engine has announced that it will shortly be changing its billing system. So this answer will be outdated within a few weeks.