For those wondering - you log in with your own account but then weirdly it says it's logged into a random e-mail address' account. I was hoping that we could just enter our own accounts and the app could simply use a workaround to fetch VIP features like good quality videos. It still works fine though and is a better alternative to paying $10 to a random person on reddit to drop you into their family. (I would nevertheless recommend BeeTV when you've logged into TraktTV which would track what you've watched; TeaTV and NovaTV also work fine, CinemaHD is alright but for some reason their website has been down for some time, and also CyberFlix TV is okay but the mod on here doesn't seem to work really well for me because the premium sources don't load, but I think that may be because my premiumize of real-debrid accounts aren't active anymore, so ... I don't know). Overall though, thanks to the modder!
Regarding the Invitation Code pop up - open movieboxpro.app somewhere (in a browser) and create an invitation code which you'll use to register from a secondary google account, then from that other account create an invitation code and use it to log in. This worked for me