How do I send an email to all users with a particular role? This was quite straightforward in Drupal 7 as I recall but, with Drupal 8  there's a lot of info I don't understand as a site builder that I am being asked to provide when creating a new rule.  

I'd be very grateful for some step by step help.

I want to send an email to all users with the role of "Officer" when a node of type "Agenda" has been updated.

Right now (Feb 10 2018) I have:

"drupal/core": "8.4.4"
"drupal/mailsystem": "4.1"
"drupal/swiftmailer": "1.x-dev"
"drupal/rules": "3.x-dev"  
"drupal/typed_data": "1.x-dev"
I'm a bit familiar with the (amazing!) Rules module. But to my knowledge the current answer to this question is a No go, that is: can't do that yet with the most recent D8 version of Rules.
Refer to #d8rules status update November 2017 to get a better understanding of the current status of Rules for D8. Here are some quotes from it:


... Fago can’t dedicate time required to develop the module further. What Rules is really missing at the moment is development capacity to help finish the last milestone.

... Even if we got more funding today, fago the current principal maintainer of the Rules module wouldn’t have enough capacity to do the work himself.



One never realizes what has been done, only what remains to be done ... So start here if you wonder how to contribute.
BTW, Rules documentation for D8 contains the official Rules documentation specific for D8.
Plan B
While waiting for Rules to become more mature, you can use the Admin Content Notification module as a temporary alternative (which does have an official D8 release already). Some details about it (from its project page):

... can be used for receive email notification when a new content of any content type will be created on your site. Although one can use rules for the same functionality but it's not good to use rules only for notification as it is a huge module, so we can use this module for small websites.

The &quot;...Although one can use rules for the same functionality ...&quot; part of this clearly does not apply to the D8 version of Rules (yet).
The image shown on the project page gives an idea of what this module can be used for.
Obviously, your question is about updates of content. But even though it doesn't say so (yet) on the project page, just have a look at the issue &quot;Also trigger event on update&quot;, which appears to be as in your question.