Many software companies have recently embraced Open Source Software (OSS) initiatives. However, volunteers (i.e., those who contribute to OSS in their spare time) still represent a wealthy workforce that drive many open source projects. Such volunteers face well-known barriers when attempting to contribute to OSS projects. What is still unclear is how the problems that volunteers face transcend the problems that employees (i.e., those hired by a software company to work on company’s OSS projects) face. In this paper, we characterize the contributions made by employees (developers that work for GitHub, the company) and volunteers (developers that use GitHub, the platform) into OSS projects maintained by GitHub (the company) at GitHub (the platform). By mining an extensive body of activities performed into five well-known company-owned OSS projects, we found that volunteers are responsible for opening more than 90% of the issues and for integrating/closing more than 50% of the pull requests of the analyzed projects. Interestingly, most part of the volunteers’ contributions are related to reengineering (e.g., refactoring activities), while employees focus more on management (e.g., updating documentation). When it comes to the working hours, we found that contributions are made mostly from 9am–5pm, even for the volunteers. Projects owned by companies can take advantage of the results of this study to understand how the external community gets involved with the project.