When someone does @bors r=abc, the bot should check if abc is a valid (GitHub) username (or a GH team?). To avoid situations where the name of the reviewer is mispelled, which happens quite often, and is problematic for https://thanks.rust-lang.org/.

It is common to specify names separated by commas, so bors should understand that and check all usernames separately.

We might also include some kind of opt-out, where you really want to use r= for a non-existent GH account.