Had this in a group call yesterday.
We had a couple people in and I added another person, but they weren't visually showing up.
https://cdn.discordapp.com/attachments/203617530827374592/203619966354849792/unknown.png

I could rightclick his "avatar" (empty space) though and had options.
https://cdn.discordapp.com/attachments/203617530827374592/203620032352092160/unknown.png