Collaborative writing is a process that allows multiple students to develop a single written text together through peer scaffolded interactions. With the increasing use of digital tools in collaborative writing tasks, new ways of uncovering information about the interactions are available for teachers to use for assessment. While computer-mediated collaborative writing has been extensively studied, there has been little research beyond assessing the product. To bridge this gap, this study implemented a set of computer-mediated blended collaborative writing tasks. The tasks undertaken online and in-person were implemented using Google Docs in a multilingual college writing class. Students’ collaboration could be traced and understood through the collaborative texts, audio-recorded peer talk, student online interactions and interviews. As part of a broader research, this study conducted content and thematic analyses, guided by the categories of peer engagement (Storch, 2001), which further contributed to defining notions of equality and mutuality in interaction patterns examined in the larger research. The results reported in this chapter suggested that peer collaboration in process writing differs in various stages and spaces. The differences present opportunities and challenges in collaboration assessment, as students were engaged with each other’s contributions, making decisions and forming a dynamic collaboration process leading to different L2 learning opportunities and outcomes. The chapter concludes with practical considerations on surfacing and assessing collaboration in a blended design.