Hi
I see it as a great advantages if the assignee list could be grouped. E.g. by groups, security level or project roles.
I use jira to manage issues or questions towards several companies (splitted into different sequrity levels). For each company there is several jira-users. When I create an issue, or should assign a jira issue, it would be a great advantage for me to have the assignee list grouped.

Assignee:
Company_A

• user 1
• user 2
• user 3
  Companty_B
• user 4
• user 5

I have serached for a plugins for this functionality, but havne't found any

If this will be implemented, as I see it, this should be setting pr. project. Project admin should be able to choose if th assignee list should be grouped by group, security level, project roles or not grouped.

An other solution would be to add security level to the first screen when creating an issue (together with project and issue type).
Then the assignee list on the 2nd screen should only contain users according to the security level set for the issue.