At the moment Invite calculate the salution for the start page from the displayname (split by space and use first part of the string).
The displayname is not always formatted as "Givenname Lastname" it often contains parts like external, team, organization and so on. The fields givenname and lastname are available from profiles and it is no problem to use one of these.
Customizing of Invite is only possible through changes directly in the deployed ear file. So changes are not persistent over updates.
It would be great to have an option to copy these html files to the customization folder and change it there, so changes are persistent and use the same functionality as all the core applications of Connections.
Or make the salution as a config option, so the administrator can decide if the startpage welcomes the user with
Hello Mrs./Mr. Lastname