Which user do you mean?
Which user do you want to select and what are you going to do with that selected user?
You can use association itself for the connected entities à First you can get the registration entity using a data grid and then you can access the associated users through any Data resources.
Or
You can create a data resource inside the course with micro flow as a source, do your retrievals in the micro flow with the passed course argument and return the user/users .