Joining a meeting can be done in two ways.
One way is to navigate to "Join Meeting" option on the navigation bar at the top.
Then enter the 36 character long unique meeting id of the meeting that you want to join and hit Load Meeting button. This will load the meeting at shown below.
The other option is to just click on the Meeting link that you received in email when someone shared the meeting with you. That link will take you directly to this page with all the meeting details already filled in.
We do allow you to join meetings without logging into the portal first, but as a good practice please login before joining the meeting because logging in will make sure that the Meetings you join appear in your history on the dashboard and you can easily join the meeting again or keep track of meetings that you're joining in.