even if old, I see that NIS can be installed and configured easily on centos6 (see http://www.server-world.info/en/note?os=CentOS_6&p=nis for example), and so, my question is: would it be usable to share SME's accounts (users/groups) among 2 or more SME servers?
It's definitely possible to share SME's accounts among multiple linux systems using NIS. But multiple SME servers is a different issue - SME server's server-manager doesn't know anything about a subservient role. Each system will consider its own accounts database to be definitive. Lots of things would no doubt start to go wrong if you tried to set this up.