Having control over involvement in an issue is a commonly requested feature, so I've tried to come up with as versatile a way to implement it as possible. Subject gives it away, but still:

When creating a custom field for issues with the 'user' type, there should be an extra check box to flag the selected user(s) 'involved'.

This is a powerful function to add. It allows admins to:

Create a 'Reporter' field, to address the desire to assign an Author. Frequently asked for around here, though I personally would still like to easily differentiate who POSTED the issue, as well as the original reporter (if they aren't the same person). All scenarios are now possible without polluting forms with a new default field.¶

Create an 'Involves' field, which can allow selective visibility for private tasks. This addresses the very loud request for better management of customer access to issues. If the user field can also be populated with group names, a manager could easily assign each issue to clusters of externals at a time. This addresses all scenarios I've read so far requesting all types of visibility control, and is thus very powerful. Even without group selection, it's an enormous improvement to Redmine as a management tool--but I believe groups should be a priority part of this feature. (groups may already be possible--I'm still on 1.4, which only populates the drop down with individual users, even if assign-to-group is enabled)¶

Please consider this additional flag. I'll probably look into what's required to make this a plug-in as I continue to study Redmine's code, but that's not to get anyone's hopes up... =)