I have found myself getting the “Unable to join” message way too many times. I’ve already checked my Internet and that is not the problem. Some people in a clan I’m in have also been getting this issue and my friends also. I just want to know what’s happening and why.
It’s a known issue right now OP, there is a detailed response from 343 in the support forums
https://www.halowaypoint.com/en-us/forums/e0d9938206f04a598ce84f2c8cadce02/topics/halo-5-guardians—investigating-server-issues/d69bdede-a980-4c6b-8827-44aad3839949/posts