If I understand correctly, you want a trigger action to automatically add a person to a group in Yammer. This sounds like a job for Flow
In Flow, you can start with the trigger that when a new response is submitted to a form, it will run the subsequent actions.
Your next action would be to add the user to the Yammer group.
At the moment, Yammer has a connector to PowerApps and Flow, but it doesn't have the create user action out of the box. You can make the action yourself by using the Yammer REST API:
If you're new to REST APIs, @ThatAPIGuy has the videos that walk through examples of them (I learned from him).
Here's one example, but you can check out the rest of his channel:
Let me know how far you get.