You can allow your members to sign in to your community using their Twitter account.
To start the process, go to AdminCP -> System -> Login & Registration, click Create New to setup a new login method and choose Twitter. You will now be prompted to fill in some details which you'll need to acquire by creating an "app" with Twitter.
Creating a Twitter App
1. Go to the Twitter Application Management site and sign in with your Twitter account.
2. Click Create New App and fill out the form. For both the Website and Callback URL fields, enter the URL to your community.
4. Go to the Permissions tab. Ensure that Access is set to Read and Write, and then check the Request email addresses from users checkbox (it will not be checked by default). Make sure you click Update Settings to save.
5. Go to the Keys and Access Tokens tab to obtain the values you need to enter into the AdminCP.
Back in your AdminCP, fill in the form, using the Consumer Key and Consumer Secret that is shown on the Keys and Access Tokens tab from Twitter.