BuddyPress.org

The size of your site-branding (header logo) is what is causing your site-header (green banner) to be so large in height. So by adjusting the height of your site-branding, you can reduce the height of the banner and as a result your header menu will fall alongside it.

Add this to your style.css :

.site-branding {
    min-width: initial;
    max-width: 35%;
}

If you want to change your banner height to a specific value rather than letting it automatically adjust depending on what is inside it, then use:

.site-header {
    height: 120px;
}