How to join Dummy Players in Multiplayer game if no player is online?

Well yes - this is possible.

In general you should ask yourself: what makes a dummy player? Your players should feel like they are real ones right? So basically visually they should just be a player. But you can make it so that they are controlled by different logic if you just attach other scripts.

Since we are in multiplayer for example the masterclient can just spawn some more networked objects (player objects) and calculate positions for them. For all non-master clients it should then look like there are some other people in there.

If you have more specific questions please post these as such and add way more details about what you are doing. Asking for a "solution" for such a broad topic is not really answerable without details.