You don't. JIRA does not have any way to assign to anything other than an individual because it simply does not work in real life.
You should implement queues, group ownerships or associated assignees instead. See https://confluence.atlassian.com/jira064/how-do-i-assign-issues-to-multiple-users-720415922.html
What Nic said. That being said, you can do a dirty solution and create a dummy team user and have the tickets assigned to them either manually or automatically (depending on how you assign) or the far better solution which is to create a Team field and set up a field or board filter so you can still use the Assignee field as intended (or not at all as the case may be).
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