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If you want to use your own phone to send the messages to this person, then you can get various programs for your PC. Obviously, you need to be able to connect your mobile to your PC (via USB or Bluetooth, etc) and obviously it'll cost you.
Group messaging, need to create an account first (doesn't require personal information or confirmation). It is US Cellular but works for several other providers I have used it for in the past: http://usc.ztango.com/usc_wm_login.html