Ah! I see what you mean now. Well, I think you're going to need a variety of plug-ins to achieve this.
First off: WP-Members doesn't have to be set to require Admin approval of registrations - you can switch this behaviour off in it's Options panel. However, its big advantage for you is that you can define custom fields for user-data, which you said earlier you'd need.
If you're absolutely sure that all members can/should have access to the secure content, and if you already have a list of them, then you can bulk-import them into WP using something like the 'Import Users from CSV' plug-in, which I think is still around. Once that's done, then you can email them from within WordPress using something like the 'Email Users' plug-in. Word Press allows you to password-protect particular pages or posts, so you could include the password to a specific resource in your email if you wanted.
If you needed finer granularity over who could see what, then you could create additional roles and move users in and out of them as required (e.g. 'Full Member', 'Temporary Member' etc). In your case - given the need to email them - that would probably be a better bet than using Groups, because Groups doesn't provide any means of emailing group members (although I believe and hope that it is on the author's to-do list).