playful-bubblegum-c070b1.netlify.app is my default site name. When I change it to keluargain.netlify.app, when user sign up using netlify identity. The confirmation email still use old site name which is playful-bubblegum.
TL:DR I can’t sign up user with custom domain, to fix it I need to rename my netlify site to its default name.
I think this is more like a bug submit instead of support.
I did a little testing on my account and was able to duplicate the behavior you’re describing. This only seems to happen to a user who got the invite email prior to the site name change. Users added to the site after the name change get the correct url.
Rather than rename your site back to the original name, you can select the user on the user list and click the send reset password email button to generate a new email for the user. Following the reset email link in that email should get them to the site at the new url. (It did for me.)