I'm playing; just started at Christmas, and currently sitting at 20-7 while I level up some characters to get their J3 LVL 65 skills.
Here's my Friend Invite code, for 5 Energy, for whomever claims it: zxqUFkEczgMu
People are saying you can get a bunch of energy from people using codes: https://www.reddit.com/r/TerraBattle...f/redeem_code/
*edit*
Looks like 19 total energy from the referral system. 5 Energy when you use someone else's referral code for your one time use Redeem Code feature. 5 Energy the first time someone redeems your code for their one time Redeem Code feature. 1 energy, capping at 9 energy, for each subsequent person who uses your code on their one time use Redeem Code feature.