Add a warning or info box for the user/group pickers

The fields Leader / Participating members / Participating groups are using user/group pickers. The user can add other different values (e.g. an email address) without a warning.
Result: an email value can be saved with a wrong value (see bug XAMEETINGS-129)
Expected Result: add a warning or info box for these fields (similar to the additional information displayed on the user profile preferences)