To join an Enterprise workspace, your account can not be a part of any existing workspaces. As the owner of a workspace, you can remove the current members of your workspace in order to join a new one. If you would like to remove the current members from your workspace, you can do so on the Manage Members settings page of your account.
From here, simply press the three dots button next to the account you wish to remove and then select Remove from team. If you remove all team members from your account, your workspace will be automatically disbanded and your account restored to a single user account. This will allow you to be able to join other workspaces as a member.
Keep in mind: You cannot join another workspace if you have an active paid subscription (even if you are currently in a single-account workspace). If you need to cancel your subscription in order to join another workspace, please reach out to support at hello@mentimeter.com.