When you join a group, you gain access to the group mailbox, calendar, OneNote notebook, and team site. Permissions are taken care of automatically so you don't need to worry about missing any meetings or conversations.
Read the full documentation here.
On the Home tab, select Browse Groups.
Enter a group name in the search box or scroll the list to find the one you want.
If the group is private, a request will be sent to the group's admin, who can accept or decline the request.