If you are using Lync inside Office 365, it is very easy to enable your users to communicate with other Office 365 users as well as Skype users. You need to be an Office 365 administrator to perform this function. Log into the Office 365 admin center and navigate to the Lync administration section. Once you are there, choose the Organization tab, and then set the value in the drop down to your preferred setting. You have three options:
1) Off completed – you can only use Lync internally to your Office 365 tenant in this case.
2) On except for blocked domains – this is the most liberal settings. Your users will be able to communicate with anyone (and anyone with your users – including seeing basic availability), except for domains you have blocked.
3) On only for allowed domains – this is the same as #2, except in reverse. You can set up your blocked or allowed domains at the bottom of the screen.
Finally, You can also turn on the communication with Skype by checking the check box under Public IM Connectivity.