This ethnographic study examined students' opportunities to learn in linguistically diverse mathematics classrooms in a Canadian elementary school. I specifically examined the contextual change of group work, which influenced opportunities to learn for newly arrived English language learners (ELLs). Based on analyses of video-recorded interactions, this study revealed a shift in these ELLs' opportunities to learn from when they worked with teacher-assigned peers to when they worked with friends. In both settings, ELLs tended to be positioned as novices. However, when working with friends, they accessed a wider variety of work practices. In friend groups, ELLs were occasionally positioned as experts and had more opportunities to raise questions and offer ideas. In contrast, when working with teacher-assigned peers, ELLs tended to remain in the position of being helped. In some teacher-assigned groups, interactions were characterized as authoritative, and ELLs' contributions and ideas were rejected or neglected without relevant justifications or mathematical authority established by their peers. The findings contribute to ongoing discussions on group work and friendship in linguistically diverse classrooms.