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When the Web App is enabled for an organizer's account, then all attendees who join their sessions have the option of joining via the Web AppInstant Join app. When it is disabled, they must join using the desktop app or mobile apps.
If you are an organizer, you can check to see if the Web App is enabled by logging in at https://global.gotomeeting.com and going to Settings. You will see the Web App section, along with a check box that enables and disables the Web App for your meetings.
Note that if you are an attendee joining someone else's session, then it is up to the scheduling organizer of the session to enable the Web App for their sessions you (even if you are also an organizer yourself).