When you go to the view of group members (by clicking the link like "3 Members"), you'll be able to select a member then delete them. If that is what you are doing, but it just doesn't delete, then update to the current version (2.7.18). Definitely more stable than the build you have as .16 had some various issues. If you still have an issue after updating, log a ticket please.
Regarding group member data storage... no. But there are a couple of db changes. I assume you are asking so you can just replace files instead of an actual install. You should always do the actual install... which replaces files anyway. Just backup any modified files.
Nathan, we are planning to upgrade
we create a custom export scrip
that called
Specific data from certain tables
And wanted to know If we would have
To redo that or that the upgrade would affect
That script.
Ok, realizing that you are on Joomla 1.5... you are fine. There are no database changes from 2.7.16 to 2.7.18. In Joomla 2.5 there ARE changes, but not in the 1.5 version.