There is no built-in function for this, you would need to use beforeStore code field to first get all groups form user, then all users belonging to each group and their emails and at the end concatenate this values into string and fill some field with this value. They you could use this field in email as To(fields)

I have created a textarea field and filled it dynamicly by a « sql query » : it works.

Here is my query (all my targeted groups are childs of the usergroup id 100. That's why I have add this condition in my query)

SELECT `#__users`.`email`
FROM `#__users`
INNER JOIN `#__user_usergroup_map` ON `#__user_usergroup_map`.`user_id` = `#__users`.`id`
INNER JOIN `#__usergroups` ON `#__user_usergroup_map`.`group_id` = `#__usergroups`.`id`
WHERE `#__usergroups`.`parent_id`='100'