The Interest and Desire parts of the AIDA model go hand-in-hand: as you're building the reader's interest, you also need to help her understand how what we're offering can help her in a real way. The main way of doing this is by appealing to her personal needs and wants.

Rather than simply saying, "Sharing your story will help" explain to the audience what's in it for them:

Conviction

As often dissappointed community members, we tend to be skeptical about claims that offer to help us. It's no longer enough simply to say that sharing a story is good, for example, but readers will take notice if you state (accurately, of course!), that shared stories have helped 5 families on average, for example. So try to use hard data where it's available. When you haven't got the hard data, yet your participation request is sufficiently important, consider generating some data, for example, by commissioning a survey.