The taxon contains ~20 spieces in a single family, the Podicipedidae, So far, the intrinsic phylogeny of grebes hasn't been investigated with molecular methods. Relationships among Podiceps species have been studied by Ogawa et al. (2015). Surprisingly, the results indicated that Rollandia has to be included in Podiceps. 

Due to convergent evolution, this group of freshwater diving birds exhibits similarity with Gaviidae (loons) and extinct Hesperornithiformes. Several species are poor flyers, and two species are flightless (Rollandia microptera, Podiceps taczanowskii). 

Genus-level classification of extant Podicipedidae following AviList checklist v2025. (link) The number of subspecies is given in parentheses. 

References

Fjeldså J (2004), “The Grebes: Podicipedidae”, Oxford University Press, Oxford.

Ogawa LM, Pulgarin PC, Vance DA, Fjeldså J, and van Tuinen M (2015), Opposing demographic histories reveal rapid evolution in grebes (Aves: Podicipedidae), Auk 132, 771-786. (pdf)

Horned Grebe (Podiceps auritus) in non-breeding plumage.