Restrict Owner to List
This plugin extends the restrict_owner option to allow specifying a list of users in trac.ini.
If you have any issues, create a new ticket.
Download the zipped source from here.
General instructions on installing Trac plugins can be found on the TracPlugins page.
Enable the plugin and configure it in your trac.ini file as follows:
[components] ... restrictownertolist.* = enabled [ticket] ... restrict_owner_list = user1, user2, ...