In the case of the GBRf trip from Crewe, IIRC it was purely done "on-line" with a Facebook page and announcements put out on various forums and "gen groups". It might have been mentioned on their website but most of the people who travelled on it were likely to have found out by one of the aforementioned means or by word of mouth from somebody who got the information from those sources.
TBH I would not say there is a hard and fast rule about where to find out about such trips as they do not crop up very often, it is just a matter of keeping your ear to the ground or your eye on the internet!
It is also probably worth keeping an eye on the tour operators such as
UK railtours ,
Pathfinder Tours , or
UK Steam for all steam trips, though of the actual operating companies
West Coast do advertise their own tours.
EDIT:
It is also sometimes possible to get "part fares" between certain stations on some trips depending on loadings but I would not recommend doing that unless you have spoken to the company concerned beforehand to make sure that they can accommodate you. By doing this of course you will need to make your own way back from wherever you get off.
