Hi so I just joined halo waypoint and halo insider a few days ago and I’m not entirely sure how to check if I was invited. can someone please give some instructions on how to check? thank you much aprecciated 
You’re too late to be invited that way, but not too late to play. If you’re on Xbox you can gain access via the Xbox Insider App, not invite or message needed. Just download the app and get yourself onto Halo Infinite through that.
If you are on Steam you will need to get a friend invite code from a friend who was invited, or a nice user from the forums. I have unfortunately already gave mine away, but there is a topic about them for users to share/request, look out for it. There is also a sticky topic posted by a mod giving out a bunch of codes.
They sound like they’re working on something for Steam so you may gain access either way if you can’t secure a code but nothing confirmed if this will.be able to go ahead yet.