My question is how do i update a column of type
"person or group" in SP.The secnario is as follows:

I have an infopath form for say, expense reports which
submits to a form lib.
On form submission,
A) if the "total expenses amount" is less than or
equal to 5000Rs, the "Assigned to" should be set to
immediate manager ,
B)if the "total expenses amount" is greater than
5000Rs, the "Assigned to" should be set to department
head.

I have a list in SP which has the the department
details specified like department heads, immediate
manager etc. Depending on who is the user(who logs
in), the immediate manager/ department head has to be
choosen.

I tried having the "Assigned To" column as part of the
infopath form for the user to fill, but the column in
SP site is not getting updated.
Note: if the colum is of type, single line of text,
then it works fine, but i have to have it as "person
or group" so as to be able to send emails, stc.