You can now export your symbols from Sketch to the Components tab in the styleguide. The Components section will include your common user interface elements and show you which screens are using these components in the project.
Exporting symbols work exactly the same as exporting artboards. Simply select a symbol or multiple symbols (which are usually located in the Symbols page in Sketch) and press Control + Command + E to begin the export process.
Once you export symbols into Zeplin, they will be listed under the Components section in the styleguide.
Generating assets from components
If you'd like these symbols to be available as an asset, you need to make the master symbol exportable in Sketch. You can learn how to do this here: Exporting Assets from Symbols in Sketch
☝️ If you only mark the symbol instance as exportable, the asset will only be available within the screen and not in the Components section.