@jksh What a bug, thanks for reporting!
Unable to add a user named in Russian letters to the group
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It just can't find such of user.
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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
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After this user is renamed himself with English, it still impossible to find and add him to the group.