If you have access to an iPad that you can use for creative sessions, Bloom HD is just one example of an app that allows people with limited fine motor control to contribute to a musical activity.
Bloom HD can be downloaded from the App Store. Other apps are available.
With Bloom HD, touching the screen produces a note and a colour. The app makes a loop of this sound and colour, before allowing you to add new sounds.
This app works well for one-to-one activities but can also be used to inspire soundscapes and group activities.