I am not sure the forums would count as a mastermind group.
As a support network sure, but a MM group tends to be more focused on helping each other reach their goals rather than discussing their goals.
It is a good place to find people with similar interests though. I am in the process of building a mastermind online right now. With people from all over the world.
Our goal is to take over the worl.. *ahem* part of it anyway is to build a big network of successful interconnected monetized websites.