form element names with multiple new rows

I have a report with many rows some of which refer to an existing
ActiveRecord model object and some new ones. I'm trying to figure out
a naming convention for the form elements so that Rails will properly
convert it into a params hash in my controller.

My first idea is to do something like the below but Rails will confuse
the data for the two 'new' rows
Enter a new invitation <input name=invite[][first_name]> <input
name=invite[][last_name]> <br/>
Enter a new invitation <input name=invite[][first_name]> <input
name=invite[][last_name]> <br/>
Update an existing invitation <input name=invite[1][first_name]>
<input name=invite[1][last_name]> <br/>

My second idea is to do something like the below and parse out the
'new_' params in my controller
Enter a new invitation <input name=invite[new_1][first_name]>
<input name=invite[][last_name]> <br/>
Enter a new invitation <input name=invite[new_2][first_name]>
<input name=invite[][last_name]> <br/>
Update an existing invitation <input name=invite[1][first_name]>
<input name=invite[1][last_name]> <br/>

Is this a common problem that others have solved in a standard way
before? Any pointers to plugins or techniques others have used?

Thanks
Alex