foo.php

<?php
class SomeModelAdmin extends ModelAdmin {
	private static $managed_models = array(
		"Member"
	);
	
	public function getEditForm($id = null, $fields = null) {
		$form = parent::getEditForm($id, $fields);
		
		$gridField = $form->Fields()->dataFieldByName('Member');
		$gridField->setList($gridField->getList()->filter('Status', 'Customer'));
    
		$fields->addFieldToTab(
			'Root.Participants',
			GridField::create(
				'Participants',
				'Participants',
				Member::get()->filter('Status', 'Participant'),
				GridFieldConfig_RecordEditor::create()
			)
		);
		
		return $form;
	}
	
}