If someone offers you "free" bitcoin or something of that nature, it's probably some kind of scam.

You may have received invitations to websites that offer "free hash power" cloud mining for signing up and for referring other users. If you get an invite to one of these websites, I would employ you to be very cautious.

These types of businesses are referred to as "Pyramid Schemes". Basically, it's where you are offered money not to actually do something but to bring in more people. So, someone else invites you and they get paid, then you invite your friends and get paid, and so on. It's illegal in a lot of places because it is exploitative.

These aren't things you want to get mixed up in.