1. Click on a collection and press Share button.
2. Choose sharing mode between 'Only invited people can access' or 'Anyone with a link can access' - just like you do it in Google Drive. If you go for 'invited people only' route, you will have to invite each person to that collection and select their access mode between 'can view' (default) and 'can edit'.
This feature is especially useful if you want to have a shared space for the whole company. Each team member can move files into such collection so everyone in the company can see these files.