Common:UseLOOKUPFunctionToGetRecordID
From AgileApps Support Wiki
Note:
You can use the LOOKUP function to return the record_id field for a record that matches specified criteria. (For the User object, the LOOKUP function returns a text string--which is what you need, in this context. For all other objects, it returns a number. To convert that numeric value to a string, wrap the LOOKUP function in a TEXT function: TEXT(LOOKUP(...)).)
