I am currently working on this part. It will basically be a “bot” that you create (really just a user account). The plugin will watch for mentions of the bot. When it sees them, it will execute the action. For example:
Bot Name: ChallengeBot
Someone types:
@ChallengeBot status
The bot will send them a DM with their current status in the challenge.
The bot will also have some other options as well.