Tools > Firm/Preparer Info and add your different preparers, each person will have their own preparer code so their info shows on the return.  You have to enter the right preparer code for that persons info to appear.

Or you each run your own copy of the software with your own clients and only 1 preparer in the list as the default preparer code.

But no, you cant assign the preparer just by the Intuit log in.