Thanks @a_5mith, let us know. The tags one definitely sounds like a new bug though.
gh#1947julianlam created this issue in NodeBB/NodeBB closed Topics with tags with an ampersand in it are not found in the tags page #1947
It just can't find such of user.
As MongoDB full text search is in beta, it has some limitations. This has been already addressed to the MongoDB devs: https://jira.mongodb.org/browse/SERVER-8423
After this user is renamed himself with English, it still impossible to find and add him to the group.