Welcome to the forum.
As you have discovered, the online journey planners will only sell tickets for up to 9 passengers in one booking. You can make multiple bookings as you propose, or contact the group travel team for the retailer you have chosen to buy tickets from to see what sort of group deal they are able to offer you.
You mention "groupsave": that is only available from some train companies and only on some of their routes.
One way of categorising train tickets is as between "Advance" tickets (which are tickets where you have to travel on the train(s) you specify at the time of booking, although there may be a little bit of flexibility if any of the connecting trains are not trains on which reservations are available) and flexible tickets (which have lots of different names, such as Anytime or Offpeak single or day single or return or day return - these can all be bought on the day of travel, or in advance, and some may have restrictions as regards the time of travel or as regards which route you can take or which companies' trains you can travel on).
The ticket you mention is an Offpeak Return: this allows outward travel on the day you specify when booking, and return travel either the same day or any later day in the month beginning with that first day. The only time restriction on this particular offpeak ticket is that it is not valid for outward or return journeys that start before 0415 (offpeak tickets for different journeys have different time restrictions). It is valid on any train company's trains. And you have to travel via Lancaster.
I had a look at whether there was any saving to be made by booking "Advance" tickets. Not every train has a quota for Advance tickets, and Advance tickets come in several price tiers: when the first tier sells out, the next tier becomes available (except that if for example there are only 8 tickets left at the price currently advertised and you try to buy 9 tickets at that price you will be offered 9 tickets at the next tier up - the solution in that case is to buy 8 and then 1). I am not sure that the timings for Crewe-Carrbridge Advances will suit you, but you may want to look at buying Advances for Crewe-Stirling and Stirling-Crewe, and then buying Advances (or Offpeak Returns once the quota for Advances is used up) for Stirling-Carrbridge and Carrbridge-Stirling (or Crewe/Edinburgh and Edinburgh/Carrbridge).
For just one person with a senior railcard, the price using Advances at your preferred times starts from about £53 return, although some of the quotas would be rapidly used up so you wouldn't get that price for each member of your party.
One final thought: having organised group travel before, there is something to be said for getting group members to pay for their tickets as soon as they are bought. In that regard, Advance tickets are non-refundable, whereas flexible tickets are refundable subject to payment of a £10 admin fee per ticket.