Actually I would agree with you that it is not a wise move to signal the gal to come over if she's sitting with another man or group, as such situations always invite unnecessary conflicts and mis-understanding.....
Lets put yourself in the other party shoes, and you will see what I meant, not just a simple matter of price-bidding but more like "I ask you to come, you must come no matter whom you are with"
Cheerios.....SS08

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Cheers SS08, agreed totally