Read more in this topic: https://meta.discourse.org/t/i-cant-get-the-welcome-message-to-display/63383?u=sidv
At first I wanted the same thing as you, that is, to welcome my users with the administrator user.
But after using a DiscoBot some time, I realized that it’s better. Because in my case, I present the bot as a “robot” that teaches users to use the forum, and I explain to them how they should do in case they want support from a real person.