When you say app, what mobile OS you are targeting? is it Android or IOS or both. Are you planning to target the tablets too ? Regarding software it is always best to have a web interface to fetch the client data, means, a website. Of course you will need internet connection to make use of app and website.

So here is the breakdown

1) Mobile app - IOS / Android / Add any other Mobile OS platform / Tablets
2) A Website (just like signinsheet.org) that can be used to administrate the data.

How much you are willing to spend ? Cost is great factor, because these days even though something looks very simple, as a matter of fact it isn't.