user nickname

Hello,

RE: notifications

How can I return a user_nicename of the user who has been assigned a post? The below shortcode returns a user ID:

User: [psppno_post meta:_pp_editorial_meta_user_assigned-to]

I want to be able to use a greeting line to the assignee in the email using their nickname instead of a userID number.

Something like: Hi [assignee-nicename], You have been assigned a new post.

Thanks for a great plugin (Planner)!