Hi,
Add a new compute field(ie, boolean field, you can hide the field)  in the form , then  in the compute function, check whether the current user belongs to particular group or not. If user belong to that group write true to the field else write false,

Then for the original field, give an attribute like this, based on the new compute field.

In the python,

compute_field = fields.Boolean(string="check field", compute='get_user')

@api.depends('compute_field')
def get_user(self):
    res_user = self.env['res.users'].search([('id', '=', self._uid)])
    if res_user.has_group('sale.group_sale_salesman') and not res_user.has_group('sale.group_sale_salesman_all_leads'):
        self.compute_field = True
    else:
        self.compute_field = False

In the XML,

<field name="compute_field" invisible="1"/>
<field name="actual_field" attrs="{'readonly': [('compute_field', '=', True)]}" />

Try this, make changes accordingly

Thanks