There are probably a few people out there that would fall in the cracks, but the best solution - in my opinion - is TWITTER. Anyone who can get a text message can get instantly updated.
The organization sets up a Twitter account, and advertises the account. Individuals also need to set up an account, then subscribe the organization's feed. Individuals set their profiles to be able to receive text messages.
When the organization wants to update people, the responsible person sends a tweet - and everyone subscribed gets a text message.
It is easy to set up separate accounts for individual teams within an organization, but individuals only need one account to subscribe to multiple feeds. So, if you have more than one kid playing, your one account gets updates from all the teams.
For big organizations like rec softball leagues, it is a great way to get the work out quickly on changes such as rainouts.
The only real cost it the cost of text messaging. You put the responsibility of signing up on the individual, so they can drop at will.