Student engagement as an important predictor of peer interaction and academic achievement has received considerable attention in second language classes. Despite its significance, how study groups engage in online writing activities in collaborative learning settings remains underexplored. To fill this gap, the present study explored how a group of four engaged in an 8-week computer-mediated collaborative writing (CMCW) project in a Chinese university English as a foreign language (EFL) context. Data were collected from multiple sources such as a pre-survey, audio-recorded discussions, and retrospective interviews. Findings identify three developmental periods of group engagement through task completion, namely breaking-in, growth, and proficiency periods. During each period, the four dimensions of group engagement (i.e., behavioral, cognitive, emotional, and social engagement) were ongoing and salient to varying extents and mutually influenced among the participants. Also, the learners utilized certain collaborative strategies to promote meaning negotiation and care for the quality of interactions. The study highlights the great potential of CMCW to form a sociocognitive learning community where the students can actively engage in learning, construct new knowledge, and promote language skills through not only cognitive processing but also mutual interaction between peers and instructors.