In js folder of pos module you can find the model.js (line no. near to 171) where you find the below code:

model: 'res.partner',

fields: ['name','street','city','state_id','country_id','vat','phone','zip','mobile','email','ean13','write_date'],

domain: [['customer','=',true]],

loaded: function(self,partners){

self.partners = partners;

self.db.add_partners(partners);

},

you can add your domain like:

domain: [['customer','=',true], ['company_id','=', USERCOMPANYID]],

 You have to extend the js file of pos.

Here you can find how to inherit the js in pos

http://odoo-development.readthedocs.io/en/latest/dev/pos/add_field.html

Hope this will help you.