We have a few posts on this topic with more detailed info, such as this one: https://community.playfab.com/questions/3843/use-shared-group-data-for-a-live-event-system.html. But long story short, no, you cannot have many players writing to the same data at the same time. Player data and Shared Group Data are not designed for that, and you would have issues with people overwriting the data, as well as throughput capacity issues on the data that could get your title throttled or blocked.
We are about to introduce a new data model which is designed for atomic data operations, and which will be used for our Guild/Team/Clan implementation. In it, you could have many people using the same data, but please be aware that update operations are not queued. If you have multiple attempts to write to the same data at the exact same time in that data system, the result will be that the first one will succeed, while the others will get "busy" responses and have to retry.
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