You can invite other users to collaborate on a styleguide with you either from the styleguide's dashboard or from the Styleguide list.

To invite a user to a styleguide, go to the right panel on the "About" page of the styleguide, then click on the "Share" button. Enter the email address associated with the user's Zeplin account or their username and click on the "+" button:

There, you will also be able to copy the styleguide's link. Those links can be shared with anyone invited to the styleguide, and they will be able to access it in Zeplin.

You can invite members from the Styleguide page of your workspace; hover over the styleguide thumbnail, click on the settings icon, and select "Invite…":