In the "Atlassian Certification Justification Letter" I found this sentence:
I am wondering how that community looks like? Is there an online forum that is for certified persons only or in what way do you take part of this community?
although I was not the Individual who designed the sentence I would say you have already found "the right place".
The community is a great place for exchange, asking questions and getting answers.
There is always the chance to involve yourself up to a level you want to - for example writing articles to share knowledge, moderate an event or probably a combination of it all.
The sentence can also be understood in a way that means you document your knowledge and you will be the recognized Atlassian expert in your company.
I can confirm there is no separate forum for certification holders. Community here is very strong though - idea is that everybody helps everybody. You are invited to contribute if you like to - everyone is welcome!