Science and Education is an elective course for all undergraduates of Universidad de los Andes, a Colombian institution. This course is designed to promote a scientific mindset while learning how to teach STEM concepts. During the third week of the course, students must identify teammates for a 3-month project. Because the team lasts almost the entire term, it is important to be able to identify complementary teammates and work effectively. To address that need, an in-class activity called Speed Grouping was designed. This study aims to analyze what are the students' reflections about the Speed Grouping activity and their process of forming teams for the project. The students' perceptions about the usefulness of the Speed Grouping activity for working effectively as a team was revised. To address this goal, a qualitative thematic analysis was conducted, using students' reflective papers based on their own diary entries about teamwork. Categories emerged from data and were linked to effective teamwork literature to analyze the level of success in forming groups for the project. The results of the thematic analysis showed that the Speed Grouping activity promoted peer selection for group effectiveness. In addition, reflections show a positive anticipation for teamwork during the project, highlighting the importance of interaction, team outcome, peers' personality and abilities, and articulation of team members.