Adding a new member to the Developers may cause a short-term decrease in your teams productivity.
When n teams are working on the same product, one person can be a member of more than one team of Developers.
The Product Owner tells the Developers how to turn Product Backlog items into Increments of value.
According to Scrum .org, Sprint cancellations are often traumatic to the Scrum Team, and are very uncommon.