Assigning a pathway gives a classroom access to it, and lets you set when it opens and when it closes.
Assigning it
Open My Library from the sidebar and click your pathway. Set the type filter to Pathways if you have a lot of content.
Click Assign in the top right.
Tick the classrooms you want, then confirm. Students see the pathway in their classroom straight away, unless you set a start date.
You can assign one pathway to as many classrooms as you like, each with its own dates.