How do I add members to a group from a list view [Firestore]?

Hello dear helpers, :)

I hope you can help me, I am really desperate :D

I want to select the users in Listview and then create a new document in the Collection Groups (a new group) with a BackendCall.

Transferring the entered name in the textfield is no problem, but I can't find a way to get the users into the new collection. :(

The goal of all this should be in the end. That Users can add a todoo via a form, select a deadline and the group they want to post the todoo to. (So right now i try to give the user the possibiltiy to create this group.) Everyone in the selected group can see and accept the todoo.

Attached is also my Firestore database

Thanks a lot