Voluntary contributions play an important role in maintaining Open Source Software (OSS) projects active. New volunteers feel motivated to contribute to OSS projects based on a set of motivations. In this study, we aim to understand which factors OSS projects usually maintain that might influence their new contributors’ onboarding. Using a set of 450 repositories, we investigated mixed factors, such as the project age, the number of stars, the programming language used, or the presence of text files that aid contributors (e.g., templates for pull-requests or license files). We used a K-Spectral Centroid (KSC) clustering algorithm to investigated the newcomers’ growth rate for the analyzed projects. We could found three common patterns: a logarithmic, an exponential, and a linear growth pattern. Based on these patterns, we used a Random Forest classifier to understand how each factor could explain the growth rates. We found that popularity of the project (in terms of stars), time to review pull requests, project age, and programming languages are the factors that best explain the newcomers’ growth patterns.