#1 - July 21, 2014, 11:08 p.m.
Once the zone is open, Nexus-Lord Donjon Rade (in Stormwind, Orgrimmar, or your garrison) will have a chat option to teleport you into the Foundry. Assemble your group, set the raid difficulty to Heroic, and then use that option to enter. Head to your right, and then another right, to find Gruul.
His model isn't final, and we know that class tuning certainly isn't final either. But this testing represents a first step towards getting there - we'll certainly be revisiting this boss again for another testing pass, so don't worry if you can't make it today or if other difficulties hamper testing.
In your posts, please note your group size and makeup (high-end guild group, PUG, multi-guild team, etc.). Uploading and linking to a log parse at Warcraft Logs, or any similar site that can handle 6.0 logs, would also be quite helpful.