Ok be warned this is time consuming...
You need to go to egov http://www.egov4you....tizens/overview and search for citizens in your country.
You are looking for ones not already in a party. Start at the top (oldest) and check each entry for party affiliation. (I always friend request every citizen in my eCountry also) When you find a citizen not in a party, message them and invite them to your party selling to them why they should join you.
Remember as you get to the younger citizens you may need to explain how to join a party. And citizens under level 13 should be invited to join once they reach level 13.
This is easier to do in a small country with a small population but harder to do in a large country like Spain.
I can only say I have found that the rewards make it worth it.