@Vincent Lauwen Thanks for reaching out to Microsoft Q&A, apologize for any inconvenience caused on this.
If you are talking about the Call Composite example (1:N call Example) that is mentioned here. I have tested this example looks like you cannot invite the other participants.
In general, As mentioned in the documentation Call Composite allows users to start or join a call. Inside the experience users can configure their devices, participate in the call with video, and see other participants, including those participants with video turn-on. For Teams Interop, is included lobby functionality for user to wait to be admitted.
Feel free to reach back to me if you have any further questions on this.