Bus service is always an option in Mexico, however Guanajuato is approx. a 5 hr. drive north of Mexico City by car. Add a bit more for a bus which will have a few stops enroute. Then do the old 'time / money' equation.
If you fly into Mexico City, you can take a taxi to Terminal Norte to catch a bus to Guanajuato.
You can also take a bus from the airport to Querétaro and from there take a bus to Gto. Depending on the time, it is sometimes faster to catch a bus from Qto to León and from there take one of the frequent (about evey 15 minutes) bus to Gto.
You may wish to spend the night in Querétaro. The historic center is wonderful, especially at night.
For airport bus info,
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