How to find out who the members of a team/group are and submit a request to join?

There easiest way to find out who are the members of a team or group is to use Outlook web app.

1. Launch a web browser and sign in to portal.office.com 
2. Click on Outlook icon.
3. Click on Groups on the left to expand the list of the groups that you belong to.
4. Click on Discover group link.
5. Search for the team/group using its full or partial name.
6. Click on the team/group name to display its profile.
7. On the profile pop-up, click on Members tab to see a list of members and their role (i.e. member vs. owner)
8. For private teams/groups, use the Request to join button to submit your request to join this team/group to the team owner.