BuddyPress.org

If you’re going to be modifying a template file, make sure you’ve created a child theme. Then, copy over sidebar.php to your theme’s directory and make your changes there.

This will ensure your changes will not break when you upgrade BP.

The “button” is simply a CSS class, so if you use the following in sidebar.php:

<a href="<?php echo bp_get_signup_page(); ?>" class="button">Signup</a>

This will give you the same look as the register button.