if u give "groups='base.group_user'" , button will be visible to any user with the right of Human Resources / Employees.
if u want to change this view for particular employee, u have to give the xml id of that employee.
For example,
<button string='Issue' name="issue_form" type="object" groups="base.group_system" class="oe_highlight"/>
by the above code, only the system administrator could view this button, as i have mentioned admin's xmlid (base.group_sysytem) in the groups.
like that if u give that particular employees xmlid, button will not be visible to anyone else.
  

Hi,

'base.group_user' is exactly any user with the rightd Human Resources / Employees. Hence, If you add this group, the button would be visible for employees (but not for portal user for example).

However, make sure that the method action_hr_email can be executed by just an employee. Otherwise, such users would see but would not be able to push the button