Hey there. The app looks really cool. Seems like a really good job on the computer vision side. UI definitely needs a bit more work - here is a couple things I noticed when using it :
(minor) Seems like you have a name clash with another stash (https://stash.ai) that went live on product hunt recently.
Some onboarding would be useful. I kind of had no idea what the app is supposed to do in the beginning. I got it after discovering a stash that I created (the aha moment!)
Push notifications : You show the requests straight after I open the app. At this point, as a user I don't know why should I enable push notifications. If I click on "Don't allow" it will be hard for you to requests it again (I will have to go to settings and re-enable it). First time is crucial. You can improve it by providing a bit more context or showing a pre-alert. More info here : http://andrewchen.co/why-people-are-turning-off-push/ and here https://techcrunch.com/2014/04/04/the-right-way-to-ask-users-for-ios-permissions/
Login / Signup screens : Shouldn't there be a modal segue in there? From the design, I can see you might have inspired those screens by Airbnb app too :) Also, screens look different - text input boxes are different. Clickable area on the back button is too small. Hard to press.
Username autocorrection : My name is Paweł (mind the last letter). I typed in my username into the box (intended 'pawel') got autocorrected to Paweł. App didn't sign me up without any error message. I'd classify that as a bug and bad UX (if there is an error in validation of the form, it's better to show it). Fix is quite simple - check autocorrection on the text field in storyboard.
Weird. Was just able to sign up using the link via an incognito window in Google Chrome. Here is the link with the invite code at the end.
https://stash.ai/signup?code=74cf41a3be484579a4821ee4661ad780
I am not sure if there are geographical restrictions on the beta either so that may be something to look into.