Most of them you just sign up and start posting crap. The details do vary, though.

A common (solvable) issue with decentralized P2P is that you only see and talk to people you've expressly connected to. Scuttlebutt falls into this category. Other's act more like a very tiny sever with very very few people, like Mastidon.

This is addressed with the concept of federation. Many tiny servers connect together, bridging into a big ecosystem, like Mastadon's federated section. Other times, P2P systems have a centralized/more-centralized area, where people can similarly meet and connect, like Scuttlebutt's pubs.

Hive/Steemit on the other hand, is a social media system on a blockchain. It costs a fraction of a penny to post/comment. When you upvote someone, they are paid crypto. To make this work out best, you want people to start with a account with a couple bucks in it. You usually buy your account and if you get serious you buy more crypto to boost your account with. Some people can make a good chunk of money posting, apparently not me though. Yet, there are programs to help onboard users for free, usually involving signing up with some affiliate partners. It's an interesting concept, but my biggest complaint is how it incentivizes viral and hype content. If a post is over a week old, the system makes it feel like garbage.