According to the AviList checklist v2025 (link), the taxon contains 22 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). The results indicated that Rollandia has to be included in Podiceps. 

Genus-level classification of extant and recently extinct (red letters) 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.