I added a non-Gmail address to my Google Group, made it an owner and then, stupidly, changed my Gmail to a Member (not an owner anymore). Now I can't control the Group membership and I can't log in with the non-Gmail address because it's not a Google account. How can I get back into my Google Group?
Add the non-Gmail address as as the primary or secondary email address of a Google account.
To add it as a primary email address you might create a new Google account non including Gmail. To add it as a secondary email address you might use an existing Google account having a Gmail email address as the primary email account.
Once you do the above, then go to you group. You should be able to access and manage it.