Put a link to your steam group in your server's thread in the Rust Server subsection?
Here and on the Steam forums (and whatever reddit is doing with server ads.)
You're also not understanding what I'm after.
It's not about advertising, It's about people seeing it and being able to easily access it.
Links don't work in game.
Hardly anyone looks at server ads, even more so after actually joining in game.
Edited:
I think I saw mention of a plugin that gave periodic chat message, like rules reminders or announcements or whatever, but I don't know the details. And if there's no plugin for that I imagine there will be soon.
You could just mention it that way. Every 20 minutes or whatever interval, "Join our Steam group, _name of group_" and searching for the group would find it easily if the name was unique.
Do this already. While it works, We all know people are lazy.
I'm just searching for ways to make it easier.
Edit: Off topic but why not allow URL's in chat ?