Dropdown to Checkbox

Hello.

I'm trying to pass every selection in a dropdown field to checkbox, but I didn't succeeded. Everytime that I select a value in the dropdown replace the first value for this last.

This is a task form and I want assign the task to several users. I don't want put directly a checkbox(multi-select) field because the user list is long and I prefer a dropdown selection, and every user input must be added in a checkbox field and selected it. In the dropdown field I want include groups of members too, after the group selection must be added every user of the group in the ckeckbox.

I don't know if I explained well.

Could you help me please?

Many thanks