How to add sortBy (use sort order of field in other blueprint) to groupBy?

Well, then you have to eitherread that order from the blueprint or create an array of values in the same order and assign a sort number to them, then sort by that array.

Basically, you would first use map() to assign a sorting order to the grouped collection, then use this assigned sort value to sort by.

Or you sort your registrations first by that grouping value, then group, like here: