Custom field of users in a specific group

How do i create a custom field that allows you to select only users that are a member of a specific group?

My primary objective is that they can assign only to users of a specific group which I know how to do using the property fields. However, I need for the user to select who they want to submit the request to when they create - so, I want them to be able to select this value on the create screen so that on save, the assignee is updated based on the selected user. That way, it is not a two-step process. We have issues with users remembering to submit when this is the case.

So, I have a group that has 4 users in it and want a custom field that allows them to select only one of those 4 group members.

Any ideas?