Since Lemmy is kind of like a bunch of small separate Reddits, its very possible that a community you are looking for already exists; its just on a different Lemmy server. But fear not! You can be the person to connect it so everyone here can see it and start to interact with it!

Use this site to search for other communities that might not yet be on this instance: https://lemmyverse.net/communities

You should find a good handful of communities around D&D, Pathfinder, Fate, etc. When you find one you are interested in, you can visit it directly, but since you probably don't have an account on that server, you won't be able to post or comment there until you connect it to ttrpg.network by subscribing to it.

Mouse over the link (it should pop up with "Click to Copy"), and copy it. Then come back here, open the search page, and search for the link you just copied. You should get a result that you can click on, but notice this time you are viewing it through ttrpg.network, so you can interact with it with your account here. Click "subscribe", and everyone here will also start to see posts from that community.

Congrats! You have just made one link in the Fediverse, connecting everyone here to a community somewhere else.