Context In GitHub, an issue or a pull request can be assigned to a specific assignee who is responsible for working on this issue or pull request. Due to the principle of voluntary participation, available assignees may remain inactive in projects. If assignees ever participate in projects, they are active assignees; otherwise, they are inactive yet available assignees (inactive assignees for short). Objective Our objective in this paper is to provide a comprehensive analysis of inactive yet available assignees in GitHub. Method We collect 2,374,474 records of activities in 37 popular projects, and 797,756 records of activities in 687 projects belonging to 8 organizations. We compute the percentage of inactive assignees in projects, and compare projects with and without inactive assignees. Then we analyze datasets to explore why some assignees are inactive. Finally, we send questionnaires to understand impacts of inactive assignees. Results We find that some projects have high percentage of inactive yet available assignees. For example, 66.35% of assignees never participate in the project paperclip. The project paperclip belongs to the organization thoughtbot. In the organization thoughtbot, 84.4% of projects have more than 80% of inactive assignees. We further observe that the main reason for developers being inactive assignees is that developers work for organizations and automatically become available assignees of some projects in the organizations. However, these developers do not work on projects. 37.25% of developers that we have surveyed agree that inactive assignees affect open source software development (i.e., causing unresolved issues or pull requests, and delaying software development). Conclusion Some organizations should improve team management, and carefully select developers to become assignees in projects. Future studies about assignees should be careful to perform data cleaning, since some available assignees are added by virtue of their employment and do not really work on projects.