I will tell you the best way to get the free ingress invitation code quicker. You can be the first one to get the invitation code among your friends and you can do some showoff for few hours or you can share the activation code with your friends by this way you can start play with your friends and build your empire by joining Enlightened or Resistance factions.
Go to google search, and search for get free ingress invitation or type get ingress activation code and it will show some appropriate results. Now Don't select any links as those links are not published those posts recently. All of their invitation codes may be ended up and they may be empty handed guys! Soldout!
So, you need to filter the search based on recency. You can do this by clicking on More and no you will see many tabs like "The web" "any time" "sort by relevance" etc.
Click on "anytime" and select "Past 24 Hours" from the dropdown list, No you will have the most recent posts, which will give you the free ingress invitation or activation code. I never joined the game, If I join, I will give you the code but by that time, you will find one for you by following this smart way to search in google! Check my Image, you will understand: