To mod someone on Twitch, you can use either the /mod command, or open the user's profile inside your chatroom.

ViewerLabs, a prominent Twitch viewer bot, has been shut down owing to legal action. However, if you’re seeking an alternative strategy to increase your Twitch viewership, you have plenty of choices.

You can "Whisper" on Twitch to privately chat with someone using a chat command or by going to their profile page, depending on your device.