That string is in the Core and can be edited via the Core's language tab option. However, it is a composite message and is made up of several different strings with replace options as part of them.
For the one you mention above, take a look a Core language strings 30, 31, 100, and 101
hth
talking-points.net/talkingpoints/group shows all groups, not /groups so either change the Group module's URL setting from 'group' to 'groups', or edit the menu template so that it links to '/group' and not '/groups'
The 'Top Groups' panel on the index page may not be listing groups by their creation date. I think by default its by the number of comments they have. Check the template code for that section to see what the {jrCore_list...} order_by parameter is.
No problem.
Is this a new Ning site you are importing with a recently downloaded archive? I ask because its a long time since I've done one and the last time I tried, and I think their download tool is no longer supported?