Hi!
Understood. Two comments:
1.- 'Search for users v2' output is an array of 0..n elements, I believe when executing 'Search for users' you always expect a single match, right?
2.- I am assuming the email address is stored under 'Email' property in each 'Search for users v2' output element, if not just try with UserPrincipalName instead

Please note on Select action block you need to switch input mode (click on the icon on the right)
Now, the WDL expressions inside are:
length(body('Search_for_users_(V2)')?['value'])
and
join(body('Select_MatchingEmails'),';')
Hope this helps