The patentable aspects of the system -- if there are any -- will lie in exactly how the system operates and whether it is novel (i.e., no one has done it or patented it before), which you (wisely) have chosen not to disclose. In many jurisdictions, public disclosure of a patentable mechanism either immediately invalidates your claim to a patent, or starts a time limit for filing a patent.
For an example of what a service built upon a patented system looks like, we can turn to Pandora Music. Pandora is an online radio service where users can thumbs-up or thumbs-down songs as they come on. The service builds a progressively better idea of what each user likes, and tries to play more music it predicts the user will like. The patent Pandora holds for this system is Consumer item matching method and system. The abstract, in part, reads:
A method of determining at least one match item corresponding to a source item. A database of multiple items such as songs is created. Each song is also represented by an n-dimensional database vector in which each element corresponding to one of n musical characteristics of the song. An n-dimensional source song vector that corresponds to the musical characteristics of a source song is determined. A Distance between the source song vector and each of database song vector is calculated, each distance being a function of the differences between the n musical characteristics of the source song vector and one of source database song vector.
In short, each song is scored according to multiple features like "uses guitar" or "tight vocal harmony" or "has a mellow feeling". Using the user's approval or disapproval history of past songs, the system tries to select a song whose features are similar to the user's expressed preferences.
The point here is that a simple idea like "A system that allows multiple users to add notes to a musical score simultaneously" is probably not patentable (just as "A music service that lets users express preferences and then selects music accordingly" is not patentable). A specific precisely-defined algorithmic system like "A system for resolving conflicts in a sheet-music document based on simultaneous inputs from multiple users" might be patentable. (However, note that edit-conflict-resolution is a heavily-explored area, and you would need to ensure that your specific system of conflict resolution has not been implemented or patented by someone else already.) The body of such a patent would go into precise detail about how your novel conflict-resolution system operates, and would certainly need to be drafted with assistance from a patent attorney.
Ultimately, a patent attorney will be able to tell you whether your idea has any patentable components, will help you search for prior art that might invalidate your patent, and will help you draft up and file any patent as appropriate.