for many new forums, the hard part is to acquire users. if there is a "Invite" button in the header when user click it it allow user to input a few email address and a message , and invite those user to register. that will be very useful. if nobody work on it, I have interest to give a try. if anybody can give me some advice which plugin i can follow that would be great.
Definitely give it a try! Utility plugins aren't as flashy as some of the other plugins out there, but you never know... 
Backend you might use src/emailer.js, front end, you can just use JS to add a button anywhere you want
Try using this library for starters: https://github.com/NodeBB/nodebb-plugin-quickstart
good luck 
For quickstart plugin. Much easier than starting with a blank slate!
We might turn that into a yeoman generator if there's demand...
UP this, i think is really interesting.
totally agreed, this is must have for large forum community
