When somebody starts a meeting with you directly, or from a room you are a member of, you will receive a notification prompting you to Ignore or Accept the meeting (Figure 1):
Figure 1 Meeting prompt
Note: Details of who has started the meeting and in what room will be displayed.
Selecting Ignore will dismiss the notification prompt and you will not enter the meeting.
Selecting Accept will automatically display the Main Meeting window and you will be present and active in the meeting.
If initially you ignore the notification prompt but wish to join the meeting later, you can select the Join button if the meeting is still active (Figure 2):
Figure 2 Join
Note: Once the Join button is no longer present, the meeting has fully ended and you will not be able to join.