ServiceReef does not allow you to manually add or register someone for a trip. The reason is that the actions of a participant need to be tied to a specific individual and for them to create an account record. If an account could be created for them or they could be added to a trip without an account then there's a significant challenge to give them access to that account in the future. Additionally, we feel it's the role and responsibility of the individual to manage their own account and actions.
NOTE: You do have the ability to move a participant from one trip to another, but they still first have to complete an application on their own.