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Re. groups - don't you have a link to "find users" above the members of the group? If you have 200+ users, it will not show all users in a select box to save on resources. Clicking on the find users link and choosing criteria will allow you to find the users satisfying that criteria and then you can add them to the selected group from the search result page.
Likewise, when you mail to users by group and have loads of users, the webserver memory can be exhausted. I have tried to optimise the resource use in the upcoming 2.0.9 beta2, but until then, you may have to find other ways to mail all your registered users. There are some threads on this issue, but I'm afraid that is all the help I can give at this moment.