You can share notes, tasks, and document collections to a group from the sharing panel on the item's detail page.
To share a note, task, or a collection of files with other people, you need to share them into a group. Here is how:
Create or join a group: go to People - Groups. If you have a subscription that allows it, create a new group and invite the people you want to share with. They will receive an email invitation and need to accept it to become members.
Open the item you want to share: navigate to the individual note, task, or file collection (you cannot share an individual file).
Use the sharing panel: on the item detail page, find the "Group Sharing" part of the settings panel above right. It shows a list of groups you belong to, where the content can be shared. Check the box next to the group you want to share the item with. It will be shared immediately.
Permissions are applied automatically: once shared, ordinary group members can view the item. Members with "content editor" rights can also edit it, or share it further into other groups.
To stop sharing, uncheck the same box in the sharing panel. Members who only had access via that group will lose access immediately.
Note: sharing an item (Note, Task) or a file collection also shares all the sub-items under it (sub-notes, sub-tasks, sub-collections, quick notes, tasklets etc). Such a sub-item cannot be "unshared" directly - if you do not want it to be shared, you need to either unshare the parent item, or move the sub-item "under" something which is not shared.
Note: you need to be a content editor (or group admin) of the group to share content into it. Ordinary members can view shared content but cannot share new items, or change the sharing status of shared items that do not belong to them.
Note: if you "own" the content (i.e. you created it), you will retain access to it even after it gets unshared from all groups that you belong to.