Exciting times. This is a subject close to my heart!
Given you’re not using a bespoke platform what you really need to focus on testing are the ‘golden paths’ – the functionality that people will use most regularly.
A good approach is to list those functions (e.g. registering a new account, making a post, replying to a post, changing your username, sending a PM etc) and get each person to perform them, recording any issues/concerns/required changes as they go.
The most important thing is to carefully document each step of the process, so that if something needs to be fixed, your dev team can replicate the issue.
I’d put together a shared Google doc that everyone can access. Then I’d get your team to compile a list of those golden paths, and get each person to perform every one of them – people all do things slightly differently.
The trick to successful testing is empathy. Try and think like your users.
People get bored pretty quickly when they test, and they tend to gloss over things. For that reason you need as many people as possible on the testing team. One person only does things one way, and that’s not thorough.
If you have other questions, fire away.