I see many of this url indexed by google, i'm not a specialist but i think it's not good for SEO and crawl budget.
If you just make search on google with : ?wpforo=signin
You'll find many url indexed.
And it's not just signin page, but google index many post with "?wpforo=signin" in the end of URL.
How can i resolve that?
you can not resolve that. We've already made many changes in URLs with GET variable, if you check the Login/Register and Logout Menus you can see it has rel="nofollow" attribute to restrict Google index. It seems there are other places where it finds and indexes. We'll chck it again and solve this issue in future releases.