(I'm having a headache currently so, I hope you don't mind some garbly sentences.)
As of writing, I've looked into the communities available on lemmy, most are pretty much dead but for a few reasons. I will not focus on that now, the objective I have for this is to explain how we could change that.
As much as you want to do community work, I advice against making yourself in a position where you have to be active all the time/regularly. It's very, very easy to get burnt out this way. In a very much online community forcing yourself to do something is counter-productive. Plus you have better things to do.
You can do this once in a while, improving things bit by bit, and let people enjoy, they will come and go.
Every community has to have a wiki. An "About", "FAQ", "Examples of what to do in this community"/"What to write about" can help people give them a choice. Not to mention a "Rules" section would be great once the community is bustling with activities.
Optionally you can add something like a matrix server, so within your community people can bond together and become friends. (Especially important for video game communities, to enable them to play games together)
And then you can further upgrade the community with adding something specific to them. For an example if the community is about writing, perhaps you can setup an own plume/writefreely instance. If the community is a multiplayer mmo game like minetest or veloren, adding a game server would boost the activity up a notch.
Lastly, not all communities are created equal, some are based around a conversation, asking for advice or help; Nothing much can be done for those stuff. However, for everything else it has potential.
Before any social platform was popular it tried to attract as many users as possible in some way. I'm not an expert so I feel like we need to discuss about this more, since our lemmy is still pretty much an infant.