In Slack Actions and Notifications Guide it has a place stating:
User mentions: type an open angle bracket, the at symbol (@), the user’s Slack member ID (see below how to find the member ID), and a closing angle bracket.
However there is no where below that mentions how to find the member id. Is there an easy way to find member id?
Open a user’s entity, and hover over the public Id in the top left. Press alt to see the UUID, and click to copy.
and this uuid can be used for @ messaging people on slack?
Sorry, what I wrote is how to get the Fibery user ID, my mistake 
To get the slack member id do this:
Click on a user name within Slack.
Click on "View full profile" in the menu that appears.
Click the ellipses (three dots).
Click on "Copy Member ID"
I can see why you think the user guide is confusing.
Thanks @Chr1sG. Sounds like I need to add their slack ID’s to my fibery database so I can automate grabbing this info then. Was hoping there was a way to automate this since Fibery has their email address and that can be used for @ messaging.
Unfortunately, I don’t think knowing their email address is enough for Fibery to be able to find them in slack. I can see that this is annoying, but yeah, adding a ‘Slack ID’ field to the user database, and doing a one-time population is probably simplest.
I haven’t checked if slack’s API allows this process to be automated, so if you had 100s of users, it might be worth looking into.
Just adding a link to the Fibery user guide about this (as this is the first result in my search):