If the external sharing feature is activated for a workspace, users with the role "Managers" can invite external users by inviting them using their Microsoft or Organization account.
To invite external members, navigate to the team tab in the settings of your workspace. Click on "+" (plus icon) and the dialog to invite external members will appear.
In the dialog, enter the email address of the user(s) you would like to add to your workspace. The dialog will point out that the found user is outside of your organization. Confirm with a click on "Share".
The invited person will receive an invitation email with the link to the Seamless Workspace.
When opening the link provided in the email, the following website will open. Choose "Microsoft account" if you have signed up for it. Choose "Organizational account" if you already use Office 365 or another Microsoft service.
The user is now registered as a member and can start working in the Seamless Workspace.
The external user will be listed in the team tab of the workspace settings. Managers can grant additional access to the user or remove him from the workspace.
If a guest user is receiving errors at first login attempt, they shall please refresh the browser after 1 minute and the Seamless Workspace should load properly. This is due to the fact that in the background, SharePoint needs some time to setup the external account and permissions.