Microsoft Looks Into Xbox LIVE Sign-In Issue
According to the Xbox LIVE support site, both Xbox LIVE Core Services and Social and Gaming services are "limited." Apparently signing into Xbox LIVE and joining other Xbox LIVE members in online games are both experiencing problems, though Microsoft has said they've resolved the matchmaking issue with Titanfall and Forza 5.
Unfortunately, signing into Xbox LIVE still remains a problem on Xbox One. We'll be sure to let you know once the problem is fixed.