Gru -
Be advised - the Screenwriters' Guild of America IS NOT the same as the Writer's Guild of America (WGA).
The Writer's Guild is the union for tv and film writers. It was set-up to protect the rights of writers. It requires that those producer's who are signatories pay writer's a specified minimum amount, royalties etc., sets acceptable working conditions, arbitrates credit disputes, etc. You need to sell a script or have done rewrites, etc. to be eligible to join.
The Screenwriter's Guild appears to be an organization geared toward wannabe writers not working writers. There are some well-respected script doctors and story analysts who they refer from their site. But I don't know what the criteria is to be endorsed by this organization - it could be as simple as those who pay to become members are automatically endorsed.
I have not heard of coverscript so I can't help you there. Just wanted to point out that the Screenwriters Guild is not what you may think it is.