It's not. In any version.
The assignee field is, by definition, a field that holds one single user account. Groups can't go in there because the field holds users. Not groups.
You are trying to do "assign to group", which was discussed recently at https://answers.atlassian.com/questions/248664/assign-issue-to-group-in-jira
We use the "Managing Issues via a User Account" option provided in Nic's link, and it works very well for us. Of course, we do still see individual users in the drop down along with the groups we have created, so we have the option of assigning to a group or individual.
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