If you're trying to assign to multiple people, then you'll need to populate a table or collection variable (i.e., multi-value) rather than a dropdown (single value with multiple options).

A dropdown will hold all the values as options for selection on the form, but only the selected value will actually be saved to the variable so that won't get you where you want to be.

When you have a multi-value field like a variable inside a collection or table, you can use that in the user field of the task, provided the values are something Forms can associate with a registered user account.

However, I've never done that with a dynamic team filter, only direct user assignment.