Ahh, so its more of a tagging system. Interesting. Are any other clients using it for groups? Are there any clients that are using both categories and Apple’s method? I will do my own research for this when I get time (Christmas!) but maybe you know off the top of your head.
I don’t think its easy for FastMail to support because our groups are discrete objects with a unique ID and various properties that allow them to be used for different things (spam whitelists, distribution lists, etc). There’s potential for groups and their members to become decoupled if the name of a group changes. Duplicate names present obvious problems. There might also be fidelity issues if a user has multiple CardDAV clients accessing the same store, though we already have that problem.