I think you're misunderstanding the GPL. From the top of the Preamble section:
> When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for them if you wish)
You are free to modify Grommunio's freely available source code and remove any user cap limits, as long as you make your source code changes available.
In fact I'm fairly certain that Section 10. "Automatic Licensing of Downstream Recipients" protects your right to make this modification:
> You may not impose any further restrictions on the exercise of the rights granted or affirmed under this License. For example, you may not impose a license fee, royalty, or other charge for exercise of rights granted under this License, and you may not initiate litigation (including a cross-claim or counterclaim in a lawsuit) alleging that any patent claim is infringed by making, using, selling, offering for sale, or importing the Program or any portion of it.
I’m curious where you think this price model isn’t allowed under the GPL.
https://www.gnu.org/licenses/agpl-3.0.en.html
(I am not a lawyer.)