Type /f perm [Faction Name] mem build yes You can also type /f perm to see all of your faction's permissions.
...no... but thanks guys hes allowed to edit now but after that one of my members betrayed me and griefed us...
Try /f promote PlayerName. This will promote them to Member, allowing them to build in your faction's land. Also, you can do /f promote again to make them an officer (Kind of like a moderator of the faction)