Using the new invites app and non iPhone users are having issues signing in

Using the new invites app. I set up an invite and sent “text” messages to everyone with the link to the invitation.

Non-iPhone users are saying they cannot sign in because it asks for an iOS account. The other problem is, I sent text messages to invitees so why is it asking for the invitee to sign in with an email address and iOS account?

I asked the invitee to use the phone number instead, it says you will be notified by email therefore you must use an email. Why?