Setting up Milestones and then associating issues with those Milestones is a great way to group issues and give them a deadline.
To set up Milestones, go to the Projects & Issues page found in the main navigation, then click the settings cog next to the project title.
In the right sidebar on the settings page, you'll see "Milestones". Click that and then add a milestone. 

Now you'll see a Milestones tab pop up next to the Issues dropdown. You'll now be able to associate issues with the milestone when you're creating or updating an issue.