I hope that I am not going to cause excess work for the bug team in putting this out. But ...............
Have the person that sent you the invite to go back to the website they sent it from (http://ghost-recon.ubisoft.com). Have them send a bug report and include your UPlay SN/GT and email address. Please try to refrain from excess banter, there is a lot that their bug team is working through right now.
I'll make it easy for you, type:
"I have sent an invite to the GRW beta to my friend and they have not received access, as of yet. My friends details below. Thank you in advance for your help in resolving this issue.
Uplay SN/GT:
email: (if you have it available. I dont know if its necessary)