People use a standard IRC client for this. Greeter-bot scripts are also a thing. They're also strongly recommended against, as nothing says 'I'm lazy and don't care about my viewers' like relying on a bot to do so. If I see one of these, I leave immediately and never come back.
Likewise, viewers tend not to like being arm-twisted into interacting with a caster by being greeted before they even say anything. Let them speak up first and say hello; at that point, giving them the option to sneak in and lurk, without having a spotlight of attention and silent peer pressure set on them by the caster welcoming them by name as soon as they load the channel.