How to create an index to improve this query
There is no way to test this without your actual data. Theoretically, your optimal index for this query will be on:

agency_id, employer, amount, service_years


This index should allow immediate push-down filtering on agency_id, followed by a stream aggregate instead of the hash aggregate by employer, and the last 2 are just to cover the query, and avoid lookups to the main table to fetch amount and service_years which the query needs for the aggregates.

Generally speaking, when designing indexes, you should consider more than one query, and see if you can find common query patterns that will allow you to create near optimal indexes that will serve multiple types of queries.
The way too common indexing strategy of 'Let's create an optimal index for every painful query' tends to lead to over-indexed databases, which will pose a set of challenges of its own, including modification overhead, and over-sized optimizer search trees. 

Let us know if the optimizer uses this index efficiently.