For the first time in my entire life I've found myself in the position of popular playground ringleader turning away the unpopular. I have no idea how to handle it, having always previously been in the unpopular turned-away position, or the keeping-heads-down-in-gang position.
I have a lot of friends who sing fairly seriously, though relatively few of them are professionals. That's relevant because they are highly motivated, but are also extremely busy with other things - so they won't say yes to everything they're asked to be part of.
I started talking to a few of them about maybe getting together for a few concerts next year to do some specific pieces that I've always wanted to do. It's difficult, very exposed music where wrong notes, poor tuning, or voice parts dropping out will wreck it for everyone.
The word got around a bit, and people have started asking if they can be involved in the project.
Two very good friends have each asked if they can join in - they are members of the same groups as I am, and very good friends of mine, so I know what they're like. One is always too busy for rehearsals and ends up learning the music the day of the concert, no matter what or when the concert is. Thus, she never really knows the music well, and sings it badly because she hasn't had time to get to know how to sing it well. She could do well if she had time, but she probably never will find the time. The other is assiduous in turning up to rehearsals and learning music, but just honestly sings really horribly - very flat, nasal tone, which affects her ability to sing in tune. She's a lovely person to have around in every possible way other than her singing voice.
They are two of my closest friends, but I would find it so much easier if they weren't involved in this, and so would everyone else. Unless they both change from what they've been doing consistently for the last 15 years, they'll mess up something that could be really good.
How does one say tactfully "no", or alternatively, tactfully, "only if you learn the music/ learn to sing in tune"? 