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. Click on Add if you want to trigger them.
Note: This option will only appear if there is at least one existing subscriber who is a member of the trigger group but has not been added to the workflow.
If this option doesn’t appear, it will be due to one of the following reasons:
All members of the trigger group have already triggered the workflow.
The trigger of your automation is something other than When a subscriber joins a group.