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MisterB,
I am fairly new to XOOPS as well, but I can give a general answer to the topic as a whole because I'm working on a non-profit site where I need to do the exact same kinds of things.
At this point, there doesn't seem to be an easy way to modify the Membership Registration to do what we want, though customizable registration is planned for the next major release of XOOPS.
So far I haven't found a separate module for membership administration either.
Within XOOPS you can assign members to groups, and then assign different groups different priviledges throughout the site.
Removing the "Register Now" from the Log In block should be a matter of going into the Administration -> Templates and finding the right block and editing it. Please note that before you can edit templates in the Templates administration section you must clone the Template Set, edit the cloned set, and choose the cloned set as the default set in Administration -> Preferences. If you do a few searches in the forums, you will find more about how to do this.
Additionally, since you are a beginner like me, please note that the templates you see in the Templates administration are stored in the database, even though there are file versions within the XOOPS folder. You cannot make changes to the files in the templates_c folder. You have to make the changes within the Templates adminstration so that the database is updated.
Anyway, I am planning to build my own membership administration module in the near future, but it could be three months before I can complete it. If you're interested, we could talk about what it should do.
Hope someone with more knowledge sheds more light on this subject, including a timeline for the next version and what kind of membership administration features it will have.
Thanks!