Collaborative writing through wikis has become increasingly popular in recent years. When users contribute to a wiki article they implicitly establish a co-authorship relationship. Discovering these relationships can be of value, for example in finding experts on a given topic. However, it is not trivial to determine the main co-authors for a given author among the potentially thousands who have contributed to a given author's edit history. We have developed a method and algorithm for calculating a co-authorship degree for a given pair of authors. We have implemented this method as an extension for the MediaWiki system and demonstrate its performance which is satisfactory in the majority of cases. This paper also presents a method of determining an expertise group for a chosen topic.