Using Entity reference + Views to create members select list
The view will need to know which group, which means a Context Filter.  So options there are usually the current url or the current signed in user.  Depending on where your entity reference is, and how you are getting to it, there might be a context available to fill your filter.  Then the view will be usable to limit the options in your reference field.  The more details you can provide about the use case, the better I can make this answer.