I have suspicion a Mac user has visibility to my text messages Can someone I don't know, set up their Mac to receive text messages from my iPhone 4?  I am having issues sending text messages; I send to a group and not all recipients receive the messages.

I do not have iMessage turned on, never have and I've never had any issues with texting until recently, everything has always worked perfectly.  I've recently changed my iTunes &amp; iCloud passwords due to the suspicion.
The short answer to your question is: No.

The actual longer answer is that if that guy with the Mac happens to work in, infiltrate or hack your cell provider or one of it's suppliers, and work their way into the SMS message system - then yes, it could be theoretically possible.

The cost of doing so, the risk of getting caught, the punishments connected with getting caught and the extremely low return of being able to read your group text messages means, that this is probably not the case.

You would probably have to be just as crazy as the guy actually doing this to be able to think that this is the case.