Someone tried to use my form and it asked them to log in.

They happened to have an account so they logged in and it just took them to their forms.

Could you please let us know what exact link they have tried to use when they were asked for logging in?

If you had simply copied the link to the page you were on while editing your form, that link is only accessible by you.

If you wish to share links to your forms, please make sure that you are using a direct link to the form, as accessible for others. You can get this direct link by clicking on Setup & Embed > Embed Codes, and copying the link that is just under "Your form is here" text:

Alternatively, if this has happened on an embedded form on your website, please let us know the link to the page where the form is embedded, so that we can take a look into what seems to be wrong.

Our forms do not ask users to log into their JotForm account, so this sounds like a possibly badly formatted link being used instead of the direct link to your form. Please let us know how it goes when you use direct form link.