I am experiencing exactly the same problem.
I invited my wife, who already has a BT Mobile sim, and when she clicks on the link on the email it gives her the message "Your invite can not been found" etc. I've tried recalling the invite and resending, but so far without success.
Also the web pages are not at all friendly, navigating through them is not easy, it took me upteen attempts to find my way to the invite page, and even more to work out how to recall and resend the invite.
I'm starting to regret moving onto the Family SIM - it really is not fit for purpose.
Solved! Go to Solution.
Twenty days on and still no success. We've been onto BT Mobile but without any progress at all. If anyone else is considering combining their families mobiles onto the Family SIM plan THINK AGAIN - DON'T DO IT.
I finally got sorted, after over a month. From what I have been told the tech teams had to physically make the required changes to force the changes to progress. It also sounds like the fault we had was bounced between different technical teams.
The agents I spoke to were all very pleasant and seemed to be trying their best, but progress was very slow.
I think the Family Sim is a great idea, but the process seems to be riddled with faults and systems that don't appear to talk to each other. As it stands I wouldn't recommend this to anyone.
I have exactly the same issue. My wife has had no access to the network for a week now. Sounds like an absolute nightmare. I echo your words. Don't bother inviting anyone to your family plan, it just doesn't work.