Can someone please advise if there’s a feature (or workaround) for the following use case:
As an Executive participating in a Discourse forum I need to grant access to my assistant but I don’t want them appearing as a separate member of the groups I belong to. But I want my assistant to get notices and be able to help me with my content.
You could give your assistant your login details. There is no other work around that I can think of.
And you could make the email address for the account be one that goes to both you and the assistant, or use a filter to forward incoming mail from Discourse to the assistant.
It’s still messy, though, as you won’t know which of you made that setting change that screwed up the forum.
It seems more honest to admit that your assistant is assisting you. 
It’s not about honesty but more about a committee (group) that is meant to convey who actually sits on the committee and doesn’t have these added names of assistants cluttering it up. If I have a Finance Committee as part of my non-profit and it has six high profile members then it should show the six members and not also include the names of six other people (who aren’t members but assistants to members).
Is this not pretty common a use case for nonprofit membership communities?
You’re the expert there. Even with a plugin, though, I don’t see much of a solution to this other than sharing credentials, which is messy. My guess is that few people will pay attention to the group membership URL and that you could have a “clean” list of the official members (perhaps with their assistant’s names?) in a pinned topic.
Wait! How about you have a Group_Assistant’s group that also had access to the category? That solves the access and notification part, but does not solve the “help me with my content” problem, though, unless you make your content a wiki, or the assistant a moderator.
I actually think the Group_Assistants group is a great workaround - thanks!
For outgoing communication, we have had the assistant post and then have a moderator follow up and change ownership of the post later.
If the assistant were a moderator, they could do that themselves (and even author the post as a PM first so it already has the executive author when it becomes visible).
Might also be worth using something like LastPass. It takes away part of the liability of giving out personal credentials.
We do this and it works just fine. We have lots of one person groups in our system for giving individuals access to private categories. Then the assistant also has access to the email account where they can also send emails on behalf of the ceo etc. In a pinch moderators also help with changing authorship of posts.