How to register a new user on XMPP using (a)Smack library

Smack has InBand registration functionality that can be used via the AccountManager class. Note that not every server has this feature implemented/enabled.

Maybe I am late, but if you are using latest smack-android:4.1.0, you can try below code for creating XMPPTCPConnectionConfiguration's connection object and register a user:

// Creating a connection
XMPPTCPConnectionConfiguration connConfig =
        XMPPTCPConnectionConfiguration.builder()
                .setHost("myHost.com")  // Name of your Host
                .setSecurityMode(ConnectionConfiguration.SecurityMode.disabled)
                .setPort(5222)          // Your Port for accepting c2s connection
                .setDebuggerEnabled(true)
                .setServiceName("myServiceName")
                .build();
AbstractXMPPConnection connection = new XMPPTCPConnection(connConfig);

try {
    // connecting...
    connection.connect();
    Log.i("TAG", "Connected to " + connection.getHost());

    // Registering the user
    AccountManager accountManager = AccountManager.getInstance(connection);
    accountManager.sensitiveOperationOverInsecureConnection(true);
    accountManager.createAccount(username, password);   // Skipping optional fields like email, first name, last name, etc..
} catch (SmackException | IOException | XMPPException e) {
    Log.e("TAG", e.getMessage());
    xmppClient.setConnection(null);
}

Just elaborating on what Flow has posted. AccountManager class has all the ingredients for maintaining user accounts in XMPP

Assume you have a connection object created.

AccountManager accountManager=new AccountManager(connection);
try {
    accountManager.createAccount("username", "password");
} catch (XMPPException e1) {
    Log.d(e1.getMessage(), e1);
}