Originally Posted by JJeffrey

Yes, as always you're not really "linking" the PNRs but rather just getting companions seated next to you for no charge. You can do this for as many companions as you wish and the # of PNR's doesn't matter nor does it matter that their return flights may be different.

However there might be issues with this when the separate PNR companions are in basic economy (I'm not 100% sure). Sometimes BE doesn't play nice in scenarios like this.