You can do that with a few clicks with sfdx-hardis VsCode extension :)
You can see details in the following article
Disclaimer: I work in the company providing this open-source plugin
You need to check Email Alert Settings,
for that, Go to Quick find >> Search Email alert. then you can find the Email alert list under Workflow & Approvals Section
If that 2nd user already having salesforce account. You can add that 2nd user in Recipients settings in Email alert which having that Email Template. Then that 2nd user also get that ...
As the previous answer to a similar scenario shows, you can leverage what the documentation goes over under Semi-Joins with IN and Anti-Joins with NOT IN
You care about User and GroupMember in this scenario. As mentioned before and noted in the GroupMember docs, you'll be ignoring "other" ways users can be part of a group with this approach.
You can't get the user names directly from that query because the GroupMember object is a joining object between User/Group and Group. You need to do another query on the User object to fetch the Name.
SELECT FirstName, LastName FROM User WHERE Id IN: userIDsFromGroupMemberQuery