You have been invited.
Pretty much this is how it works.
You need to train people to mine for ore. that is how you make your cash. After that, you must train people to defence, offence, spies etc. You can also upgrade weapons for everyone. The thing is, one higher cost weapons is much more effective then 15 lower cost items. You can also upgrade houses to produce more people a day etc. I am still learning this game myself, I just started playing it today.