We present the discovery of two galaxy overdensities in the Hubble Space Telescope UDF: a proto-cluster, HUDFJ0332.4-2746.6 at z = 1.84 ± 0.01, and a group, HUDFJ0332.5-2747.3 at z = 1.90 ± 0.01. Assuming viralization, the velocity dispersion of HUDFJ0332.4-2746.6 implies a mass of M 200 = (2.2±1.8)×10 14 M , consistent with the lack of extended X-ray emission. Neither overdensity shows evidence of a red sequence. About 50% of their members show interactions and/or disturbed morphologies, which are signatures of merger remnants or disk instability. Most of their ETGs have blue colors and show recent star-formation. These observations reveal for the first time large fractions of spectroscopically confirmed star-forming blue ETGs in proto-clusters at z ≈ 2. These star-forming ETGs are most likely among the progenitors of the quiescent population in clusters at more recent epochs. Their mass-size relation is consistent with that of passive ETGs in clusters at z ∼ 0.7 − 1.5. If these galaxies are the progenitors of cluster ETGs at these lower redshifts, their size would evolve according to a similar mass-size relation. It is noteworthy that quiescent ETGs in clusters at z = 1.8 − 2 also do not show any significant size evolution over this redshift range, contrary to field ETGs. The ETG fraction is 50%, compared to the typical quiescent ETG fraction of ≈ 80% in cluster cores at z < 1. The fraction, masses, and colors of the newly discovered ETGs imply that other cluster ETGs will be formed/accreted at a later time.