create-the-field.php

<?php

// Hook into the BuddyPress registration form
add_action( 'bp_after_signup_profile_fields', 'add_honeypot' );

/**
 * Add a hidden text input that users won't see
 * so it should always be empty. If it's filled out
 * we know it's a spambot or some other hooligan
 */
function add_honeypot() {
	echo '<div style="display: none;">';
	echo '<input type="text" name="oh_no_you_dint" id="sucka" />';
	echo '</div>';
}

validate-the-input.php

<?php

// Validate the input
add_filter( 'bp_core_validate_user_signup', 'check_honeypot' );

/**
 * Check to see if the honeypot field has a value.
 * If it does, return an error
 */
function check_honeypot( $result = array() ) {
	global $bp;

	if( isset( $_POST['oh_no_you_dint'] ) && !empty( $_POST['oh_no_you_dint'] ) )
		$result['errors']->add( 'pjbp_honeypot', __( "You're totally a spammer. Go somewhere else with your spammy ways." ) );
		
	return $result;
}