When adding another individual, or individuals to a Special Interest Area project you can either nominate that you are doing the project with them/you are working together on the project with them, or, that they are a mentor or someone who will support you along the way.
When adding them as an individual within the Special Interest Area part of the achievement pathways, they will just be written there as text, not added as a co-collaborator. If the youth member is undertaking this as a Project Patrol, then they can add other youth members (from their Unit, or Group, or by membership number), through this functionality.
It’s important to remember that Special Interest Area projects should have individual goals for each individual that is undertaking the project.