Yep, It should be possible for multiple users to share an account.
Multiple people should be able to safely log in to the Team A account at the same time.
Yep, I believe that multiple people being logged in is standard functionality, so a “Regular User” account should work perfectly fine.
I think that SugarCRM Pro gives the option to only have one person logged in to an account at one time, should you require it.
However, I ran a quick test there by logging in to a “Regular User” account on 3 different browsers and creating records, it appears to work fine.