The root cause of this error is that LD have restricted a small number of attributes that you can do calculations on for potential performance concerns. Imagine if the number of people in that group ran into thousands. Then it would take a fair length of time to do the calculation and the architecture might not be able to handle that and hence errors would be seen downstream. To prevent this, the system.groups attribute, and others, are set so that you cannot do calculations.
So while the LD reasoning is academically correct, it does introduce potential issues for someone wanting to do something perfectly reasonable like yourself where the number of real life items in the collection is small. Personally I'd ask your local support team for their recommendations. There may also be some clever brains who contribute other calculations that don't hit these limitations.
I would strongly recommend that you avoid modifying the metadata for that attribute to allow calculations as you may end up hearing the sound of polite hand washing from the LD support team at some point in the future, especially when you upgrade.
You could also consider voting here as I'm sure the developers could remove this restriction if there was sufficient demand