Howdy,
One more time, with feeling. 
I am using Jira Atlassian Jira v8.5.7 and I am a sadm. I asked a similar question here...
https://community.atlassian.com/t5/Jira-questions/Can-t-Add-Specific-Role-to-Filter-Shares/qaq-p/1648127
...but now I'll expand my question and use some pics to help explain the issue.
I have a project for which I am an administrator/project lead/owner. In this project, I have 5 roles set up, one of those is Users...

The role of Users exists in my Project Permissions, where I've assigned Users under the Browse Projects permission...

Now, when I go to any Kanban board within this project, and try to Edit Filter Shares and add Users to the Filter Shares (in this case I want to add Users as a Viewer of this board), Users does not appear in the drop down of Project Roles...

I will pause here and ask... What am I still needing to do to be able to add Users in this fashion?
Here's an additional oddity, in other, older boards associated with this project, I was at one time able to add Users under the Edit Filter Shares > Viewers...

However, if I open these particular Filter Shares, there appears an error in red re: an Unknown Role. If I delete Users from the Viewers list, the error goes away, but again I am left not being able to add Users to a Filter Shares as expressed above.
Again, what do I need to do to add Users to the Filter Shares > Viewers, and it would be very helpful to understand why this Unknown Role error is showing.
Thanks!