If that field is required to be read-only, you can do that simply by clicking read only property in the property of the control. Incase you want field to be edited by few persons you should create data source in the form and get the datasources filled by attaching them to the dataconnection. You can use 'GetUserProfilebyName' webservice to fetch the names of the people who can edit the field. Now you just require to put conditional formatting on the field, that would read as if 'login user is not equal to datasource1 and login user is not equal to datasource2 .....' then disble the field.
Let me know fi you have some questions or concerns.