Yes, completely. There is no fee to sign up, and no fee for anything a couple needs to do on the site.
For free, you can browse every vendor, read reviews left by other couples, save the vendors you like so you can compare them later, and message any vendor directly. You will never be asked to pay to find someone or to get in touch with them.
The vendors are the ones who pay to be listed. That is what keeps the whole thing free for you.