Doing /res friend add [ign] would add a player to your friend list on towny. that friend can have a separate perm from the resident/ally/outsider . so you and your friend can build on your plot if you ever wanted to share your plot with a specific player. you can do /plot set perm friend [build/destroy/itemuse/switch] [on/off] to set there perms on your plot.
Hope this helped!
note! plot perms need to be individually set