Although, as mentioned above, the official modding tool won't be available for Baldur's Gate III until September, Larian wants to make sure that Patch 7 for the game arrives as polished and stable as possible, which in this case meant running a series of tests over the summer.
The first closed alpha test of the mod tool has just started today, as Larian says in an community update on Steam. In this test, the studio has selected a handful of modders to work side-by-side to review the major new features in Patch 7, while a second phase of its plan will be a closed beta in July with around 1,000 users, which (thankfully) anyone can sign up for.
For answers to all questions about the upcoming beta and how the modding system works in Baldur's Gate III, you can check out the full FAQ on the Discord channel here.