As a guest user, you're completely anonymous on our servers. Ideally you nevertheless personalize your profile with your name and a handsome selfie so your friends and family know which posts came from you.
After you've already joined your first event tap the profile icon in the lower right corner of the app to access your profile, then tap the gear icon in the upper right corner.
From here you can open your user settings to change your nickname and your profile photo. You either snap a selfie within the app or upload an existing image from your library.