Purchasing one of the new Early Access Supporter Packs with a key will grant you access.
If you have bought a top-tier supporter pack for $480 USD in the past or have spent $480 USD or more in total purchases in Path of Exile on a single platform, you will automatically gain access to Path of Exile 2. In this case, you will receive an email and a PM on the website informing you that your account has been granted access.