Anyone can read. To post, vote or mark an answer solved, sign in with the account your community host gave you. Your password goes only to that server; the forum only gets a proof of who you are.
No account yet? A member can send you an invite code: join here.
Posts are public: anyone can read them. Leave out passwords, keys and personal details.
Where community accounts live (a Matrix server) and where the forum is hosted. Set once by whoever runs the community host.