sample rows of subgroups from dataframe with dplyr

Yes, you can use dplyr:

mtcars %>% 
    group_by(cyl) %>%
    slice_sample(n = 2))

and the results are like this

Source: local data frame [6 x 11]
Groups: cyl

   mpg cyl  disp  hp drat    wt  qsec vs am gear carb
1 24.4   4 146.7  62 3.69 3.190 20.00  1  0    4    2
2 26.0   4 120.3  91 4.43 2.140 16.70  0  1    5    2
3 21.0   6 160.0 110 3.90 2.875 17.02  0  1    4    4
4 17.8   6 167.6 123 3.92 3.440 18.90  1  0    4    4
5 14.3   8 360.0 245 3.21 3.570 15.84  0  0    3    4
6 15.0   8 301.0 335 3.54 3.570 14.60  0  1    5    8

Historical note: slice_sample() replaces sample_n() in dplyr 1.0.0 (May 2020). Early versions of dplyr required do(sample_n(., 2)).