You don't. The assignee is the one single person responsible for an issue, and it should stay that way (because multiple assignees simply does not work in real life. It has to be a single person with ultimate responsibility)
See https://confluence.atlassian.com/display/JIRA/How+do+I+assign+issues+to+multiple+users for the ways to indicate group involvement
Hey Community mates! Claire here from the Software Product Marketing team. We all know software development changes rapidly, and it's often tough to keep up. But from our research, we've found the h...
Connect with like-minded Atlassian users at free events near you!Find a group
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no AUG chapters near you at the moment.Start an AUG
You're one step closer to meeting fellow Atlassian users at your local meet up. Learn more about AUGs