Multiple new task emails generated for different time zones.

Can you try unchecking the "Override with initiator's timezone" option in the general property tab of the process model which hosts the send email process. By doing this we are making it as a static timezone so that the process can be used across multiple timezones.

Can you specify how many users are in the groups?? Because the new task email seems to be sent in some certain batches like for 30 people something like that. We noticed similar behaviour in our application. All people wont be copied in the first batch, others will be in second batch and so on.