There are a dozens of tips to earn money without investment or inviting friends.But I will suggest yousome best ways for earn thought that way.
They are -

1. Bounty and Signature In this forum you can earn a huge money from doing bounty tasks and Signature campaign.
2. Freelancing Another big opportunity to earn money by doing Freelancing job.This is the most popular and profitable too if you have real skills.
3.AirdropsFrom a legit Airdrop you can earn huge money.