Which version of Sage CRM are you using?

CRM Lead Data.xlsx

This is the file I am using. It is a bunch of made up/random test data.

Have you tried using the userid for Susan Maye instead?  I'm not sure it will even let you.

I have tried 'Susan Maye', 'MayeS', and '78441508327527' (the user_CRM_SID). None of them work.

'Susan Maye' does not return an error for the 'Assign To' field. 'MayeS' and '78441508327527' return errors about an improper data type for the 'Assign To' field. The error is:

Here is the latest version of the spreadsheet I am trying to upload.6038.CRM Lead Data.xlsx

I'm not sure it is necessarily relevant if you are not able to import an integer but you would have to import the user_userid field.

There is an obvious workaround here which is to import the userid into a separate integer field and then get a table script to update the assigned to field.  It's not a fix but it would be a quick and easy way to get your data in correctly.

Thank you. I agree.

Another work around is to upload them with out assigning the lead, then edit each lead and add the assignment. That is easy to do when I upload 5 leads, but what about 500?

This still doesn't answer, why doesn't the 'Assign to Users:' field work?

Or you could import then with the 'Make a Group' checked and then do a mass update afterwards to set the Assigned User.

Also what have you set in the 'Match User Fields' drop down when mapping the field?
There is an option for 'Firstname Lastname"

78441508327527 will be incorrect, you're not looking for her SID number, but her user_userid number as per the user table, I can't remember the demo data off the top of my head but will be a 2 digit number at most. 

You can then import against this, SID is a 'session ID' not a user ID so the import wont understand it. 

I have used this concept before for live imports, importing the user's ID to the lead_assigneduserid, making sure I select that is the option I using as well in the mapping options

78441508327527 will be incorrect, you're not looking for her SID number, but her user_userid number as per the user table, I can't remember the demo data off the top of my head but will be a 2 digit number at most. 

You can then import against this, SID is a 'session ID' not a user ID so the import wont understand it. 

I have used this concept before for live imports, importing the user's ID to the lead_assigneduserid, making sure I select that is the option I using as well in the mapping options