Skype for Business Online helps people in your organization keep in touch. You can also let users add people outside your organization to their list of Skype for Business contacts. Go to Admin > service settings > IM, meetings and conferencing > external communication to turn this feature on or off.
When you turn on external communication, users can add Skype for Business users in other organizations to their Contacts list.
Keep in mind:
The other organization must also turn on external communication.
All Skype for Business Online features are available when you IM or do computer calls and desktop sharing with these users.