I would recommend that you have a look at setting up and IRC server at your work place or with a free service such at Freenode.com
. Connectivity isn't really going to be a problem as most every computer/smartphone should be able to connect as long as the port is open. Not being a Windows user very much I don't have a lot of experience with Lync so I can't really help you there.
As for client side Pidgin is an option it isn't that hard for people to setup just a couple of menus and they are done. The one problem that I have had with Pidgin is that if I use one username with pidgin then I can't use it agian on another client. As for the people that are a little more tech savvy I might recommend mIRC
, and for you have a look at thier help pages
. This will give you some insight on setting up and running an IRC server. You could even set one up in a test enviroment and see if you liked it.
Well that is my five cents. If you have any questions I can help where I can.