Sketchfab is all about community. Beyond the option to comment on models, Plus members (and above) can interact with their followers or fans through the Contact button on their profile.
Once you click Contact, you'll be asked for your name and message. After completing this form, Sketchfab will send your message in the form of an email to the Sketchfab Plus (and above) user.
If you use the Contact button, your email will be revealed to the user you're contacting. You will not see their email address until they reply.
If you'd like to contact a Sketchfab Basic user, the best option would be to comment on one of their models. By default, the user should receive an email notification whenever someone comments on their models.