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In order to share set of Scheduled Issues just select them and press the Share Selected link:
Use the drop-down menu cloaked under the "Projects" (1) to access a desired one and then go to the below located "Scheduled Issues" (2).
"Share Selected" (4) button becomes active when you tick (3) particular scheduled issue(s) from the list above it.
You will see the window that is similar to the window displayed after choosing Share single Scheduled Issue action:
The main diiference difference is that you can choose between overriding current sharing settings or joining new permissions to the existing ones.