Please look at this response by Russell Christopher in the following thread Access to views and dashboards published to Tableau Online (not Server) . I have copied response of Russel and pasted here for your convenience.
Response from Russell Christopher
The guest account is not honored by Tableau Online.
Guest "works" on a core-based machine because core-based licenses essentially mean "all you can eat" in terms of adding users to the machine. It's a much more expensive license.
Tableau Online is very different - you're paying per user account on the service, so there is no "Guest" to speak of. Anyone who wants to access content must be a named (and paid for) user.
I think the real issue here is "pay to play". It takes resources to render a report no matter how you slice it. If you "own" the box and have paid for a core license, you can choose to allow guests to use up your resources however you want-- you're the boss of you!
In a cloud solution that uses a subscription model, it doesn't tons of sense (at least to me) to invite anonymous guests in to view & use content. You need to pay to play because those guests are using up resources that could be used for "paying customers". Mercenary, I know..