An Application Programming Interface (API) provides functions for interacting with contact lists and contacts that are stored in a unified contact store by a primary contact service. For example, a client of a unified communications service may use the API to access contact information that is stored with a primary contact service (e.g. a messaging application/service). The contact information is maintained by the primary contact service. The API includes functions such as, but not limited to: adding a new IM contact to a group, adding a new IM group, removing an IM contact from a group, adding a distribution group to an IM list, getting an IM item list, and tagging an IM contact. The contacts may be obtained from the primary contact service and temporarily stored by the client (e.g. within a cache) of a different service.