The Starter plan trial is a 14-day trial during which you can access Starter plan features (read more about them here). If you are on the Free plan (which only supports a single Editor), you can invite other Editors during the trial, but note that they will lose access to your team at the end of the trial. However, they will regain access as soon as you subscribe to the Starter plan.
If you create multiple styleguides during the trial, you will lose access to your extra styleguides once the trial ends. You will still have your first created one. You can regain access once you subscribe to the Starter plan.