Hello!
I've been testing Lync Online 2013 because we'd like to replace Skype with Lync. As always happens in these cases, this solution is viable for 99% of seats but not all: we have in fact very few users with an unsupported Windows version, thus we'd like to keep them with Skype and move the great majority of the company to Lync.
First step I made was creating a Microsoft Account for those users, linked to their company email address - let's say tintoretto@venice.com. They don't have a SIP address, nor a Lync license. Other people have instead an assigned Lync license and a SIP address (eg caravaggio@venice.com).
Lync users can chat/call with external users as long as they are not @venice.com, while if Caravaggio (Lync) adds Tintoretto (Skype), no chat is possible. Caravaggio's messages are not delivered, while Tintoretto is not...