The only way the game allows you to do this is to find the hirelings in the game, take them into your party, and then dismiss them. As soon as you do that, they make their way to the Adventurers' Inn. Of course you cannot do this until the hireling you invite to join your party is prepared to do so.
The only way to add a character who is not a hireling to the Inn is to use an editor.
I can't remember who it was (maybe it was Ice?), but somebody created two starting positions where your player character is a dragon, but you can't dismiss this character to the Inn. (I played games starting from those positions years ago. They were fun, but they made the game a bit too easy.)