@annchristine –
In general, you need to look in the wp-content/plugins folder to find the code for your widgets.
The code for the built-in groups widget is in wp-content/plugins/buddypress/bp-groups/bp-groups-widgets.php
follow up: I took a look at the BuddyPress widget pack… that gives you widgets for member (popular, etc.) not for groups.
To set the tab order in the tabbed widgets plugin, it looks like you can just do it from the widget’s admin panel… changing the tab from the dropdown.
Unfortunately, the widget broke in my install, so I can test it.