Update: It appears that the friends list issue on Xbox Live has been fixed.
Original story: If you are having problems finding friends on Xbox Live, you are not alone, Microsoft is currently in the middle of dealing with an issue that's affecting friends lists on the online gaming service. The problems span the company's Xbox consoles along with other devices, including Windows 10.
According to a message on the company's Xbox Live status page:
"Are you having trouble viewing the activity of your Friends list? We are aware of this issue and are working hard to get this fixed ASAP! Thank you for being patient while we work. We'll post another update when more information becomes available."
We will also update this post when this problem has been resolved.
Source: Xbox Live Status