3GPP has identified two architectures for network sharing. In both architectures, the radio access network is shared.
In first architecture only radio access network (E-UTRAN) is shared and is referred as the Multi-Operator Core Network (MOCN) configuration.
In second architecture evolved packet core network element MME is also shared in addition of radio access network (E-UTRAN). This configuration is referred to as a Gateway Core Network (GWCN) configuration.
I am not aware of SGW being shared in these scenarios. You may also refer to below url for a detailed writeup:
http://www.unwiredinsight.com/2013/3gpp-lte-ran-sharing-enhancements
However in National Roaming or Geographical Roaming scenario the subscriber uses SGW of the roaming network with either local breakout (PGW connected to SGW using S5 interface) or home routed traffic (PGW connected to SGW using S8 interface). Thus in both cases subscriber uses SGW on the roaming network. Some roaming is also considering as a type of sharing. So I just mentioned in the same in context. However this is not sharing between multiple PLMNs.
