So the quick answer is: yes people interested in Solarpunk are very welcome here ๐
The slightly longer one is: The server this Lemmy instance runs on is currently far from overloaded. My current guestimate is that without major changes the setup can maybe take 10x as many active users. With some optimisations I have in mind for a dedicated database server probably also a few more (maybe around 10k monthly active users???), but that's about the maximum I am willing to scale it as long as this is just a hobby project with no compensation for my time spend.
Thus if you want to further promote this instance on /r/solarpunk please try to explain that people can also make accounts on other Lemmy instances and access our communities from there. So if they are really active contributors that identify strongly with the Solarpunk movement, then by all means let them make an account here. But if they are only an occasional poster & lurker, another Lemmy instance (maybe a regional one?) works just as well and this helps spreading the server load onto more shoulders (as server to server communication is lighter on server resources).
As for moderation help: yes additional community moderators are likely needed, as not all of the current ones seem very active. Additional site admins would be also nice, but I am a bit hesitant to add people for that role that are not already longer term members of this instance and seem sufficiently trustworthy. For now, the approval of new accounts doesn't take that much time so I can easily manage it alone.