An online system defines a new group of users of the online system for a content provider to use as an audience for its content. The online system generates the audience by first allowing the content provider to specify a subset of the users of the online system, along with a weight for each specified user. The online system then performs a lookalike expansion on the specified subset to obtain a larger group of users of the online system who are similar to the specified subset. This lookalike expansion is weighted using the weights provided for each of the members of the specified subset. Once the resulting expanded audience is generated, the online system performs a content selection process for content to present to users of the online system, where membership in the cluster group is used as an eligibility criteria in the content selection process.