It’s a great realisation that you do not need to ask for sign in at the first screen! Seriously, enough colleagues in UI design simply do that unquestioned, because it seems like a best-practice. It is not, and rather frustrating for users, actually, leading to bounces.
From Apple’s Human Interface Guidelines on Accounts:
Ask people to create an account only if your app’s core functionality requires it; otherwise, let people enjoy your app without one.
Delay sign-in for as long as possible. People often abandon apps when they’re forced to sign in before they can do anything useful.
If Login is optional (which most of the time it is actually), it should never stand in the way of achieving a user’s goal. Most importantly, it should not be thrown in their face in a pop-up.
But once they finished their task, and the app’s value became obvious, they are more open to additional engagement.
The keyword here is Gradual Engagement. LukeW wrote some great articles on the topic.
Twitter's recent sign-up redesign boosted new user conversion 29% by using gradual engagement.
[…] registration is either postponed, or handled behind the scenes and the first time experience is focused on giving people an understanding of how they can use a service and why they should care to.
So confirmation pages might be a great place to motivate users to create their account.
Great, you created your first event!
_Sign Up_ to synchronise events with your other devices.
Your Sign Up option in the profile is a great idea as well, as users who actually plan to do so right away probably will find it there.