Just tried to send a message in game chat, and tag the creator of the post. Autocomplete didn't match the creator of the post, only the other player who had commented in chat.
It should probably match any player that has joined the game as well as anybody who posted in chat. Someone may have joined the game, posted a chat message, and then left the game because they had something else going on. People should still be able to autocomplete to their name to reply to their message.
You could also think about adding the ability to long-press on someone's message to automatically reference them (and anyone they referenced in their message) Sort of a "reply" capability.
