There are two places to find the Lesson Plans that have been shared with you using the View Group feature.
Calendar
From the Workspace Calendar, use the View Group dropdown to select the Teacher you want to view. After selecing a teacher, the calendar will now reflect the lesson plans that belong to them. Navigate to a Lesson Plan and click it to open the lesson plan for viewing.
Lesson Plan Dashboard
From the Lesson Plan Dashboard, there is a section labeled My View Group that lists all the teachers that allow you to view their lesson plans. Expand the Lesson Plan tree under the name of the teacher until you find the lesson plan you want to view.