Getting your CFI certificate is hard, takes time, and requires a lot of work. I know. But if you are doing that, you have already done most of the work to get a ground instructor certificate. Do it at the same time! (most people find themselves unlikely to come back and do it later)
The Flight Instructor Airplane (FIA) knowledge test database and the Advanced Ground Instructor (AGI) databases are very closely aligned. If you can confidently pass the FIA knowledge test, you are likely able to pass the AGI knowledge test also.
While you are working to get your CFI certificate, taking the AGI knowledge test at the same time, for many people, the same day, as they take the CFIA knowledge test makes a lot of sense. While you will have to sit for two separate tests and pay for each of them separately, passing the AGI knowledge test will make you eligible for an FAA Ground Instructor Certificate
You can either choose to process after you have completed your CFI, or before it as long as you have already completed the Fundamentals of Instructing (FOI) knowledge test.
Why might you care to have a ground instructor certificate you might be asking?
Well, if you ever want to have a “Gold Seal Flight Instructor Certificate,” you will need to have a ground instructor certificate.
Ground instructor certificates can also be used in many ways (click here for more information – https://www.gleimaviation.com/2019/04/19/you-should-become-a-ground-instructor/#:~:text=A%20Basic%20Ground%20Instructor%20(BGI)%20can%20provide%20the%20ground%20instruction,portion%20of%20a%20flight%20review.) and it becomes more unlikely for CFIs the further away from their initial CFI training that they do the work to get this certificate.
The same holds true for an Instrument Ground Instructor Certificate (IGI) while you are pursuing your CFI-I certificate. Take the IGI knowledge test when you are preparing for and taking the CFI-I knowledge test, and you can add this on also.
The good news is that once you have done this, there is no practical test needed. There is just an administrative issuance paperwork process that needs to be completed to issue a ground instructor certificate that many DPEs can do; avoiding a need for you to get to an FAA office to issue the certificate.