An EFIN is an electronic filer identification number issued by the IRS to tax preparers who electronically file tax returns for their clients. For a tax preparer to file electronic returns to the IRS they must have an EFIN.
To get an EFIN simply fill out Form 8633 either online thru the IRS e-services or print and mail it in. It is recommended that you submit your completed application at least 45 days prior to the date you intend to begin filing returns electronically. Using the e-services online application process may speed up the time it takes to get an EFIN.
http://www.irs.gov/Tax-Professionals/e-File-Providers-&-Partners/FAQs-About-Electronic-Filing-ID-Numbers-(EFIN)