Subscribers can be added and removed into a sequence in the following ways ;
You can add subscribers manually into a sequence list by clicking on the "Add Subscriber" under the action column as shown below;
You can remove the subscribers from a sequence by clicking the Subscriber list
You can make your customers auto subscribe to a specific sequence by just selecting the sequence from the Settings-> Sequence Auto Subscription as shown below
You can make one customer subscribed to multiple sequences as per your business needs;
In order to add a subscriber into a sequence when they message particular keyword , you can set action in the Smart Replies "Add to Sequence"
Unsubscribe customer from a sequence using Action in smart replies "Unsubscribe Sequence"