I'd like to let users specify which other users may edit their content. Basically a user would be able to create a piece of content, which they are then allowed to edit but no other users can edit (accept "admins"). This is all standard Drupal stuff so far and very easy. But now I'd like to let the user give permissions to other specific users to edit the content they created. So, the "creator" can now assign "editors", letting other users edit the content the original user created.
In case this isn't quite clear yet, here is a use case...
A site sells subscriptions which allow privileged users to create a Project on the site (Project in the sense of a software development project). The user who purchased the subscription can now create nodes of various content types that make up the content of the Project. But since no one likes to work alone, they can invite others to their Project. The invited users can edit any content created by the user who purchased the subscription.
So... how do I allow a user to specify certain other users which can edit his/her content and give those selected users edit permissions on all nodes created by the original user?