This depends on your use case.
If you are targeting a small number of users, you could have signup be by invite only. This can be accomplished by sending potential users an email from your app that contains a deeplink that has a unique code which when clicked, returns the user to the signup page of your app.
If you need open signup to your app, one signup pattern is to require a unique email for registration. You can check the email against emails already registered in your app.
If you are worried about users registering accounts with emails that they do not own, you can send a confirmation email with a deeplink which when clicked, returns to the app with a unique code and confirms that the user owns the email that was registered for signup and activates the account.
There are many methods to accomplish signup that can be very complex. The best advice is to use the simplest and most secure method that satisfies your use case.
Hope this helps!