@Søren Alsbjerg Hørup , I just did the test and it works fine for me. I added the user to my AAD Tenant as guest user and that user did receive the email from my tenant with the invitation. When clicked on the "Get Started" button, it takes me to the invitations' link and asks for the consent. Please refer to the screenshot below:
I have not used that url https://b2b.azure.net/signup?tenant, before. But try the email way and I hope that works for you.
Hope this helps.
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