Email referencing undefined data

In an email titled "[X] new ENCRYPTED messages from [person x], [person y], and Y others", the email referenced undefined users (and a negative number of others):

_"Keybase users undefined, undefined, and -2 others tagged everyone in one of your team channels in 4 encrypted messages using Keybase chat."_

Start by reproducing the encrypted-message email described in the issue and trace the code that assembles its recipient names and “others” count. Done means the email no longer contains undefined users or a negative count, including when multiple people are involved.

Written by the indexing model from the issue text.