Right at this moment its not possible to purchase more BigBlueButton users on a Free plan.
However, there is some good news. We are about to include some changes to how BigBlueButton works with MoodleCloud in the 3.4 Moodle release. This will include some exciting changes to the number of users per session included on your site and how you can get more users.
We are working hard on getting this to all MoodleCloud sites in the next 2 weeks. So If you can wait for these hopefully it will give you what you need. I can't give you the exact details yet, however there will be an email to account admins explaining all the changes, so keep an eye on your inbox.
You are able to use other conferencing software on MoodleCloud, however it won't integrate as well as BigBlueButton.