If you want to add the same note to multiple contacts, you have the ability to create a Batch Notes. You can set it up on any contact groups or on All Contacts.
Using the check box on the left side select the contacts you want to include in the Batch Notes.
The Batch button will appear after selecting the contacts, click the button and select Batch Notes.
Once you add the information, you can choose to pin the note, which means it will be on top of the activity feed, or you can also assign a follow up task with the note. Remember to hit save note to make sure the note is saved in the selected contacts profiles.