Connecting android app to server?

I am working on an android app that basically brings functionality of a website to android. What I would like to do is to allow a user to sign up to the website from the android app. I think I should a use a service but I would like to take precautions to prevent the service from being abused, i.e. someone flooding with many fake accounts. Does anyone ha开发者_如何学JAVAve any suggestions? What should I do on the app or on the service to prevent this? I though about using a captcha generated on the server but I would like to avoid that.

Edit--

I need to be able to create new user from the app, so I'm worried someone will figure out how I connect to the server and they will just start creating many new users. I am thinking on doing this with a post request but i am not sure if I should

Edit--

It just came to my mind that I should limit the requests coming from an IP address. Do cellphones share IP addresses or are they all unique?

You can use a WebView inside your mobile application, through this you don't need to get worried about authentication as you are already doing it in your web application.