When your meeting reaches the ceiling for the maximum number of participants allowed in a meeting, it will block any other guests from entering the meeting.* The guest attempting to join will instead see a "Meeting Full" message and your usage report will show that a guest tried to connect without luck. (e.g. If your account allows 5 guests and a 6th guest tries to join, he/she will be blocked from entering and will see the "Meeting Full" message.)

However, you can increase the account capacity immediately by clicking the "Upgrade or Downgrade" link on the My Account page and then selecting the size of the account/room you need.
Your account will change immediately upon clicking the Update Now button. Simply advise the guests that were being blocked to try again and this time they will be allowed into the meeting.

*The Host is not counted in the total maximum allowed.