How do I use the GETUSERID() function?

I cannot find anything that gives me the correct syntax.

What you want to do? Can you give some detail?

Thank you for your reply! I appreciate any and all help!
I was trying to use it in a Reporting Services report. I thought I could use it to get the Domain account of the user attempting to run a report. Then I want to compare the Domain account to data in a table that would then return the divisions that the user has access to and show only those divisions in a report parameter dropdown. I tried using SUSER_SNAME() but that gives me the account that the Data Source is using and that is not what I need. Is there a different function that I could use in the T-SQL code in the data set for my report parameter?