Microsoft confirmed earlier today that Xbox Live services were offline with major server issues. A recent post from the Xbox Live support team on Twitter does confirmed issues affecting core Xbox Live services.
This included problems using party chat, as well as playing multiplayer games in groups, and trying to access other friends services.
If you're running into issues with party chat, joining your friends online & signing in, or trying to view profiles, our teams are aware and on the case. We'll be sure to provide updates here when we have more to share.
— Xbox Support (@XboxSupport) January 12, 2019
Later in the day another Tweet indicated the issues had been mostly resolved and encouraged users to reach out if there were further problems:
Good news, everyone! The issues previously listed in our last few posts have now been mitigated, so feel free to give those another try. Thank you for reporting the issue, we’re here and we’re listening. https://t.co/LofAusFNrR
— Xbox Support (@XboxSupport) January 12, 2019