Changes where made (about 4 years ago) to SME to allow access from Site B, now I want to add access for another site (Site C) but I don't know where in SME to add new site.
Your question is so broad and vague, the answer really depends on what changes were made 4 yrs ago, and you are the only one likely to know that.
Probably try looking in the Local Networks panel in server manager, the link to the Manual is at the top of the Forum page, so please read that. If you had done so, you probably would not need to ask that question here.
They are running version 5.1 - and won't change
Then they cannot expect to get support for that version anymore, most people will have forgotten what 5.1 even looked like, let alone what functions it did or did not support.