How to retrieve/calculate citation counts and/or citation indices from a list of authors?
Effectively the main problem is to build the citation graph. Once you have that you can compute any metrics you want (e.g. h-index, g-index, PageRank).
Supposing you have a collections of papers (that you've retrieved in some way) you can extract the citations from each of them and build the citation graph. You might find useful ParsCit, an open-source CRF Reference String and Logical Document Structure Parsing Package which is also used by CiteSeerX and works great.