We're on Mac. We have Office installed but Teams isn't part of it; we need to install that separately. The staff doesn't use OneDrive or anything like that, I only need to set them up with Microsoft accounts for software licensing purposes. Most don't even know their Microsoft passwords. The whole office (40 ppl) is watching a webinar. The presenter wanted to use Teams. We normally use Zoom, but fine, I installed it on all machines. No login was necessary on a couple test machines I did last week. A few of the people are now being asked to log in, either with my credentials or their own. No idea why, but we really just wanted to watch a webinar and it's ridiculous to need a login unless you're presenting something or engaged in a conversation.
So, we set up the presentation on one machine in our Library space so those who couldn't connect could sit and watch it. With masks on. Fun!
Why would teams decide some people need a login and some do not? There is no difference in the "type" of user except for me (admin) and, like I said, even I didn't need to log in.