The Ingress Prime Beta (2nd Wave), initially promised to be released by private invite on August 31, 2018, has been delayed by up to 2 weeks. Niantic announced this via email to agents who registered for the beta and signed the NDA.

Niantic announced the 2nd Wave of Beta invites in a G+ post on July 19th:

X group of people will receive Prime beta access no later than Aug 31.

The Ingress Prime Beta will be open to all Anomaly attendees who registered, checked-in, and hacked the registration Portal of any Cassandra Prime XM Anomaly event and 2018 Agent Olympiad participants no later than the end of September.

During an AMA responded to on August 13th Andrew Krug promised:

Q66: During Cassandra Prime we could hack a special portal “for a chance to be granted early access to Ingress Prime Beta”. Does this mean that only selected people of those who hacked will get the early access? Or all of the people who did?

A66: A subset of people who hacked the portal will be chosen at random to be granted early access to the beta.

On August 31st Niantic sent the initial email to the 2nd Wave of Beta invitees:

Please sign NDA and create a Reddit account….your Prime Beta invite awaits you.