I published MyGPT publicly. I sent the URL to my friend who has a GPTPLus account. There is a signup button, but no login option. If he clicks Signup, it takes him to another page where he can either login or create a new account, so he logs in properly, but when redirected back to my GPT, he is not logged in and again only has the signup button available.
Note that my friend who is having trouble accessing the GPT is using an Apple login for his OpenUI account. Others who can access it successfully have logged in with Google, I believe.