Back when gmail was only available by invitation only, you could sign up for a Google account using a non-gmail email account. Which is what I did to start the Silvern Castle Google group. I never bothered to transfer ownership to my gmail account when I finally got a gmail account.
Up until a few years ago I was able to login using my non-gmail account to manage this google group, then one day I when I tried to login I got a prompt to verify myself with my phone number. This was odd since I never enabled two-factor login since I am the last person in the world still without a cell phone, so I was unsure what phone number I could have entered. I thought I might have entered by google voice number, but since I had no idea what that number was, I exited the browser as I had an appointment to make. Big mistake.
Later that day I got the dreaded google email sent to my alternate email address saying my account was locked after suspicious activity was detected. At that time the only recourse to prove I was me was to answer questions about when I created the google account (to the month and year) and when I first started using which google services on it (again to the month and year). I don't know about you, but there is no way I could answer any of those questions so my account remain locked.
After a year or two, I tried again, this time it finally asked me for the phone number again (something up to now if never ever asked except for that first time when I got initially locked-out). So I tried my land-line and got a call from the google bot with a verification code which worked to unlock my account.
As I learned, Google does not consider free gmail users customers worthy of any live support. Because I still knew the original password and am so retro that I still had the same land-line for the last decade I was able to eventually unlock the account once the stupid lockout screen actually asked for the phone number (remember, after I got locked-out it would only accept me answering dates I could not recollect). Makes my wary of continuing to use any google email or service that I may actually rely upon.
Anyway, long story short I now transferred ownership of this group to my main gmail account so I should be able to manage this group again if anyone still follows it. Whether it matters anymore in the age of twitter and facebook remains to be seen as I got off the social media bandwagon a long time ago and will not go back.
Apple II Forever
Jeff Fink