Applescript to find and add specific contacts to a group
This is one way to do get the syntax right for this task:
to addContactsByEmail:(e_mails as list) toNamedGroup:(group_name as text)
        local e_mails, group_name
        tell application id &quot;com.apple.AddressBook&quot;
                set target_group to the group named group_name
                repeat with e_mail in the e_mails
                        tell (the first person whose e_mail is in the ¬
                               value of its emails) to if exists then ¬
                               add it to the target_group
                end repeat
                save
        end tell
end addContactsByEmail:toNamedGroup:

I've also wrapped it in a handler to be called as a re-usable code block, e.g.
my addContactsByEmail:{&quot;foo@bar.com&quot;, &quot;people@desks.com&quot;} toNamedGroup:&quot;Imaginary Friends&quot;

You may wish to add some error-handling to cover the situations where the specified named group doesn't exist.  Currently, it'll throw an error (the same as yours would), but you could either elect to exist the handler gracefully if the group doesn't exist; or to create the group automatically.