I like Suse definition as well.
No maximum required, btw.
Not in theory, in the practice to many people can be a problem.
If I really want to be anal about it, I could state that an exchange of money,goods or services between the participating parties is also a requirement.
That seems to match the regular use in moderns languages. In the romantic literature, an ambiguous feeling of love between the client and the provider is frequent. Like a joke of fate or something like that.
Too many people can complicate the things even more, and lets live the job of the Book Keeper at a side.