I published that tip a while ago on Ning Creators. For those who might have missed it I think it's worth publishing here as well:

We put a category keyword in the LOCATION field of the group's home page.

Most of our groups are not location-based. Many Group Creators don't bother to fill that field, and some write something funny or clever instead. Our groups that are location-based have the location in their group name, so they remain searchable.

We use a hashtag before the keyword to let GC's know it's a reserved field.

When you click on the LOCATION field, you get a search results page that contains all the groups with the same keyword.

We then create menus for the group categories. In Manage/Tabs, nnder the Groups tab, we add as many sub-tabs as we have keywords and link them to the search results page for each keyword.

If you need more than 10 categories you will need to create an index page because Ning doesn't allow more than 10 sub-tabs to a tab (anybody knows why?)

It's a simple cheat that works well if your Group Creators don't really need to use that field.

I hope it's not a repeat =)