You can search the company directory for distribution lists or Office 365 Groups and send the IM to everyone in those groups. There are several ways to send an IM to a group. Then, click the button next to the last contact you select, type your message at the bottom of the conversation window, and press Enter. If the people you want to send the instant message to appear in the list as contacts who aren't adjacent, press and hold the Ctrl key while you click each contact. If the people you want to send the instant message to appear as adjacent contacts in your Contacts list, press and hold the Shift key while you click each person. To start an IM conversation with multiple contacts, do the following: Send an instant message to several people at once
For more information, go to Use stories in Skype for Business. You can even give it a title if you want. Note: If you have a long message, Skype for Business breaks it into sections and gives you recipients the option to read more or less of the text.
