I'm assuming by "one difficulty available" you're talking about the Easy/Medium/Hard/Expert links at the top of the band page. These are simply to mark the Highest difficulty that ANY member of your band is playing on. When you put in the individual scores, it gives the option of choosing each person's difficulty.
For example, if your band was playing on hard guitar, expert bass, and easy drums, you'd file it under the "expert" tab, because that's the highest difficulty any single person in your band is playing at. If it were hard guitar/medium bass and medium vocals, you'd put it under "Hard." Then, when you enter scores, there is an option to pick what difficulty each member played on.
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