  We present a sample of four emission-line galaxies at $z=6.11-6.35$ that were
serendipitously discovered using the commissioning data for the JWST/NIRCam
wide-field slitless spectroscopy (WFSS) mode. One of them (at $z=6.11$) has
been reported previously while the others are new discoveries. These sources
are selected by the secure detections of both [O III] $\lambda$5007 and
H$\alpha$ lines with other fainter lines tentatively detected in some cases
(e.g., [O II] $\lambda$3727, [O III] $\lambda$4959). In the [O III]/H$\beta$ -
[N II]/H$\alpha$ Baldwin-Phillips-Terlevich diagram, these galaxies occupy the
same parameter space as that of $z\sim2$ star-forming galaxies, indicating that
they have been enriched rapidly to sub-solar metallicities ($\sim$0.4
$Z_{\odot}$), similar to galaxies with comparable stellar masses at much lower
redshifts. The detection of strong H$\alpha$ lines suggests a higher ionizing
photon production efficiency within galaxies in the early Universe. We find
brightening of the [O III] $\lambda$5007 line luminosity function (LF) from
$z=3$ to 6, and weak or no redshift evolution of the H$\alpha$ line LF from
$z=2$ to 6. Both LFs are under-predicted at $z\sim6$ by a factor of $\sim$10 in
certain cosmological simulations. This further indicates a global Ly$\alpha$
photon escape fraction of 7-10% at $z\sim6$, slightly lower than previous
estimates through the comparison of the UV-derived star-formation rate density
and Ly$\alpha$ luminosity density. Our sample recovers $66^{+128}_{-44}$% of
$z=6.0-6.6$ galaxies in the survey volume with stellar masses greater than
$5\times10^8$ $M_{\odot}$, suggesting the ubiquity of strong H$\alpha$ and [O
III] line emitters in the Epoch of Reionization, which will be further
uncovered in the era of JWST.
