Go to the Subscribers page > Choose a list (or click View All next to Subscribers) > Select the subscribers you want to text > Actions > Add to workflow.
Things to Note
When you manually add someone to a workflow, they will receive your first workflow message immediately.
People can also join an automated workflow based on a list or by texting a keyword.
You can't manually add someone to a keyword workflow — they will have to text a keyword to join.