Click the main menu (on the circle on the top right), and hit "switch accounts" (it'll say "Login" if you have no accounts). You'll be able to add accounts and switch between them on the sidebar or through that page.
I was going to ask if you were planning to make an app version of Photon, but I realized that may be a dumb question since this can be installed as a PWA.
If you don't mind answering, what are the advantages and disadvantages between a native Android/iOS Lemmy app and a Lemmy PWA
I personally love the look of what you have going on here and would like to continue supporting it.