Received my email and flight message also see that I am listed under Xbox July Tech Preview in my flights. When I click through it tells me I am not invited.
Help.
Can you check your Insider profile and make sure it says Xbox at the bottom?
https://www.halowaypoint.com/en-us/halo-insider/profile
if it does, you can go here for help on how to install
https://insidersupport.halowaypoint.com/hc/en-us/articles/4404216548756-Installing-on-Xbox-Halo-Infinite-Technical-Preview