As you know Epic Games release this game (Ark Survival), i did a server and i have a problem , the people who want to Sign In to Forum without Steam cant Sign In. To join this Forum Steam is Required as i know. Is there another way to sign in?

Epic Games doesn't have an account API, nor a community system. The only way to sign in here, with the free provided Forum is to leverage another secure system through what's called Oauth2.

Until EGS brings it'self up to the standards of 2010.... the only thing you can do is purchase web hosting from someone and run your own forum.