Does anyone know if Messenger Plus contains any spyware?
That's a matter of opinion; it depends on what you call ‘spyware’.
Messenger Plus bundles an obnoxious always-on adware component (the ‘Sponsor Program’) run by ‘CiD’ or ‘C2 Media’, a company with a very shady past of deceptive bundling and drive-by-downloads. You can choose not to install the Sponsor Program at install-time, which I highly recommend, but for me it taints msgplus by association.
The Sponsor Program hides under randomly-generated and mutating names to try to avoid being detected and removed as malware, such as:
O4 - HKCU\..\Run: [DUMB SITE] "C:\ProgramData\tworoamroam.jfj7w4"
O4 - HKCU\..\Run: [SEND BALM CREATIVE INTRA] "C:\ProgramData\way proc locks.cs6cm0t"
and refuses to uninstall without an connection and captcha. The Sponsor Program is a nasty piece of work which I wouldn't run on any machine let alone a corporate workstation, but does it count as spyware? It does connect to its controlling server to report stats and download updates, but I haven't observed it leaking browsed URLs or keystrokes.
Is it relatively safe to install at work?
Without the Sponsor Program, maybe. Messenger Plus will also send back statistics about how you use the program itself, which can also be turned off.
But even ignoring the optional horribleness that you'll have to make sure the employees don't forget to disable, is this the kind of software you want running in your business? A load of wacky skins and emoticons and funny noises? I suggest it is a frivolous waste of time that can serve no purpose for your company.