library(data.table)
dat <- data.table(id=1:10, group=rep(1:2, each=5), x=rnorm(10))

> dat
    id group           x
 1:  1     1 -0.39384959
 2:  2     1 -0.03081369
 3:  3     1 -1.30571673
 4:  4     1 -1.82379155
 5:  5     1  2.36751011
 6:  6     2  0.21523454
 7:  7     2 -0.18905780
 8:  8     2  1.80707868
 9:  9     2  0.88348164
10: 10     2  0.38374826