Composite is to component as Aggregate is to __?

The answer seems to be "no". I suppose the candidate word would be "aggregant"[1], which doesn't exist.

Maybe the difference with "composite" is that components of a system are "put together" (as in the Latin etymology) in a specific way, and so there is a need to talk about them in relationship to each other and to the whole they make up. Whereas an aggregate has always been a more loosely-based grouping of just stuff that may or may not have specific relationships to each other (going back to the Latin etymology which seems to be "to flock"); it may simply be that there's never been a big need to refer to the elements of an aggregate as specifically belonging to an aggregate, as opposed to using general words for subsets of things or words that refer to those individual items specifically. Like element, component, subset, item, unit...

ETA-

[1] "aggregand" from Edwin Ashworth's comment does sound better.