No, you cannot RSVP to an event or activity within Scouts | Terrain.
You don’t need to RSVP in Scouts | Terrain, because the system does this automatically. All activities that you are invited to, relevant to your current role, will display in your calendar view in Programming. This includes Patrol, Project Patrol, Unit & Group activities.
We are examining how to improve RSVP’s and add system notifications in the next round of development.