I initially had this problem. In my case, it appears that the cause was the share invitation being sent to a different e-mail address than the e-mail address that I use for my Ring account. At first, I tried forwarding the invite e-mail to my Ring e-mail address, but that didn’t work. I then had my relative instead send a share invitation directly to my Ring e-mail address. It worked instantly.
I know how frustrating these problems can be. Hopefully the solution that worked for me will work for you. Good luck.