// Modify BuddyPress registration fields
function modify_buddypress_registration_fields() {
    ?>
    <p class="register-field">
        <label for="phone_number">Phone Number</label>
        <input type="text" name="phone_number" id="phone_number" />
    </p>
    <?php
}
add_action('bp_before_registration_submit_buttons', 'modify_buddypress_registration_fields');