Hi,

you might not 'hide' a field for a group, but you might only name allowed group. Thus, in the field 'message_follower_ids' indicate allowed groups (e.g. if is done to restrict portal users, put there 'base.group_user').

If you can't specify all the groups, which are allowed to access a field, try to prepare different xmlform views, and indicate to which groups this view should be applied. Find the example in /hr_timesheet/views/project_views.xml:

<record model="ir.ui.view" id="view_task_form2_inherited">
            <field name="name">project.task.form.inherited</field>
            <field name="model">project.task</field>
            <field name="inherit_id" ref="project.view_task_form2"/>
            <field name="groups_id" eval="[(6,0, (ref('hr_timesheet.group_hr_timesheet_user'),))]"/>
            <field name="arch" type="xml">
…

This form would be applied only to a group 'hr_timesheet.group_hr_timesheet_user'. Thus, you might replace/hide a certain field for this group like:

<field name='message_follower_ids' position='attributes'>
	<attribute name='invisible'>1</attribute>
</field>

This update would be applied for all members of this groups and its heritors.