For a collection of entities having navigation paths to related entities, adding an aggregated value from the related entities to each entity in the collection today requires a first argument to groupby listing all properties. For example, to retrieve all products together with their total sales amount:

GET ~/Products?$apply=groupby((<key property>,<all other properties of type Product...>),aggregate(Sales/Amount with sum as TotalAmount))

returns all entities from the Products entity collection whose structure now also includes a dynamic property TotalAmount. If the underlying type comprises many properties, this approach becomes rather clumsy.

Options to describe this request in a leaner way could be:
A) Do the aggregation inside an expand transformation with a nested compute argument that operates on the entities of the navigation target. The determined result is then added to the result as expanded navigation property:
GET ~/Products?$apply=expand(Sales,compute(aggregate(Amount with sum) as TotalAmount))&$select=*,Sales/TotalAmount

returns

{  
  "@odata.context": "$metadata#Products",  
  "value": [
    { "ID": ..., "Name": ...", ...
      "Sales": [{ "TotalAmount": ... }] },
    ...
  ]
}

(this option requires an extension of expand to allow applying compute to related entities)

B) Allow $it as first argument to groupby meaning that each entity of the input set becomes a separate subset on its own:
GET ~/Products?$appy=groupby($it,aggregate(Sales/Amount with sum as TotalAmount))

returns

{  
  "@odata.context": "$metadata#Products",  
  "value": [
    { "ID": ..., "Name": ...", ...
      "TotalAmount": ... },
    ...
  ]
}