It is worth mentioning that the patch uses == as first comparison, failing to equals comparison. So if you're using a constant you'll reap the benefits of the == check, that is if the two string objects are the same.
It's also worth mentioning that the GroupManager is simply an addon, it's not a vital part of the framework. In fact, if you feel that the GroupManager is insufficient you can easily make your own by extending Manager, the framework promotes that just as well as it promotes you creating your own systems.
There are many ways to implement a GroupManager. I could have had two implementations, standard GroupManager that only associated one group to an entity and a MultiGroupManager, where you could put an entity into multiple groups.
I'm pretty sure there are better ways to implement a GroupManager, don't consider the bundled version to be the holy truth
You can also create a GroupingSystem and a Grouping component, but that would be more experimental, which I'd like to know how works out.