Hi Chimera,
Have 2 list boxes in ur form.
1)Initially Listbox A contains all the names.Listbox B is empty.
2)Have 2 buttons between these listboxes.One button(>>) transfers the selected names from Listbox A to ListBox B while the other(<<)button does the opposite.
3)The user can select a name from Listbox A,click the >> button to transfer it to Listbox B.Once the user transfers a few names,he can then send mail to all the users in ListBox B.
4)To send mail to all the selected users in ListBox B,go through the above link provided by Kalyan Reddy.
U will need to write code to implement this though.
Now to ur next question,it is possible to have a repeating section containing a dropdown.The user can insert a as many rows,select an individual in each row's dropdown and finally send an email to all of them.
But if the number of users are too many, then the form becomes too long.
Regards,
Abhishek