Traditionally, scholars in physics education research have focused on students solving well-structured learning activities at the university. However, due to their constrained nature, these problems hinder collaboration and idea generation. In order to encourage student collaboration and decision making demands among undergraduate students in an introductory physics course, we utilized a real-world problem where groups were asked to compose a well-structured physics problem for younger learners. In this study we explored how they collaborated in composing physics problems. Data collection consisted of audio recording of the group discussions while they were collaborating to develop their physics problems and their respective problem solutions. Through interviews, we accessed participants' perceptions of the task and related challenges. Results suggest that composing problems is an opportunity for participants to propose ideas and make decisions regarding the goals of the problem, concepts and procedures, contextual details, and magnitudes and units to introduce in their activities. Further, participants valued the open-ended nature of the task and recognized its benefits in utilizing physics ideas into context, which in turn enabled collaboration in a way not experienced by them with traditional well-structured problems. These findings shed light on the nature of composing physics problems in undergraduate courses, where participants assume a different role by facing a real-world activity that encourages expertise through physics-related communication and writing.