QR scan or location basis for Guest-join feature
Guest-join feature must easy as an instant! Study applications like WeChat, Whatsapp, etc inviting guest through their cellphone number or QR code or email address as an account name. Microsoft Teams should focus on Industrial level of social networking, where is secure, traceable record, and easy to use!
Adding business partners can be as easy as 1, 2, 3. Open Teams, open Scan/QR code, send invitation.
Unique device login. Only one device logs in at one time. To login on tablet or computers, must confirm on cellphone or email for security. Close to quit the software.
Group. Instead of Sub-group, sub-sub-group, or topics, maybe just use one group per topic is easier to identify.