I have tried for more than 20 times to join a multiplayer game and I have not had success
It’s likely this is still related to the Xbox Live outage last night, follow this topic for the time being and keep checking the XBL status page:
https://www.halowaypoint.com/en-us/forums/6e35355aecdf4fd0acdaee3cc4156fd4/topics/xbox-live-outage-13-january-2018/327b0364-c689-40ea-b859-ef553573b918/posts?page=1