What instance are you using? The hard limit is indeed 25 users, but you can see in this doc that "The default maximum number of members is equal to the memory of the instance for that environment divided by 60 MB, with results rounded down." If you are using a nano instance, it only has 0.5 GiB and, therefore, a limit of 8 users.
To increase the maximum number of members, you can upgrade the cloud9's memory by changing it's instance type. You can follow these steps to achieve that:
*If you choose one with 1 GiB you will have up to 17 members.