Is this missing an apostrophe ?
It is very common for companies and brand names to remove apostrophes from brand names*. It is also common for us to add a possessive ('s) that doesn't exist.** This annoys the purists but you can't tell a company how to write its name. I'd say it's not "McGregor's" suite/bar/function room, it's "McGregors" because that is how they have chosen to name it.
* Barclay's became Barclays, Waterstone's became Waterstones, and so on.
** We called Safeway "Safeway's" before it became, erm, ,"Morrisons". F.W Woolworth became both "Woolworth's" and "Woolworths", and of course you have "Tesco" which is colloquially referred to as "Tesco's", not to be confused with "the Tescos" who know who they am.
Interestingly, the home of golf is "St Andrews" whereas the home of the unwashed is "St Andrew's".