Currently, it is only possible to manually add existing subscribers to workflows by using the trigger when subscriber joins a group. It is not possible to manually add any subscriber to any other type of workflow.
A workflow can only be triggered after it is activated. So when you activate a new workflow, only new subscribers will trigger it. Existing subscribers who have been a member of the trigger group before the workflow was activated will not be triggered automatically. To trigger them in the workflow, you will have to manually add them.
To manually add an existing subscriber, open the workflow in edit mode and click on the link Add subscribers to workflow on the right-hand sidebar.
You should see the number of existing subscribers who are part of the trigger group but have not yet been added to the workflow. You can choose whether to trigger these subscribers from the start of the workflow or from a specific step.
Once you've made your selection, click Select.
