Representing a domain knowledge through an ontology allows people and systems to share a common corpus of knowledge for reasoning and communicating. We have recently introduced groups to the MADCOW annotation system, and in this paper, we describe how the integration of ontologies allows more refined annotations to be posted on groups focused on some domain. In particular, concepts of the domain ontology can be used as tags, which are integral components of annotations, thus allowing more semantically significant queries for retrieving annotations on specific topics. Services for promoting participation to groups of potentially interested users can also be fostered by the adoption of domain ontologies. For example, authors of annotations systematically using tags referable to some existing ontology can be invited to join groups based on that ontology. Conversely, the analysis of texts of annotations, or of annotated texts, associated with some ontology, can represent a useful addition to construct associations between the ontology concepts and the set of lexical terms associated with them.