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

library(data.table)
dat[, x := replace(x,seq_len(.N)==.N,0) ,by=group]

dat
#    id group          x
# 1:  1     1 -0.3148360
# 2:  2     1 -0.1737918
# 3:  3     1 -0.6768283
# 4:  4     1  0.4066397
# 5:  5     1  0.0000000
# 6:  6     2 -0.3606155
# 7:  7     2  0.1965135
# 8:  8     2  0.1488247
# 9:  9     2 -1.8684589
#10: 10     2  0.0000000

dat[, x := ifelse(seq_along(x) == .N, 0, x), group][]

#     id group           x
#  1:  1     1 -0.39593612
#  2:  2     1 -0.16050994
#  3:  3     1  0.27635653
#  4:  4     1 -1.13830591
#  5:  5     1  0.00000000
#  6:  6     2 -0.50796065
#  7:  7     2 -2.08919586
#  8:  8     2  1.02132570
#  9:  9     2  0.05813759
# 10: 10     2  0.00000000

dat[, x := replace(x, x == last(x), 0), by = group]

    id group          x
 1:  1     1  0.1456084
 2:  2     1  1.1704387
 3:  3     1  0.5015786
 4:  4     1  0.1095455
 5:  5     1  0.0000000
 6:  6     2  0.5186358
 7:  7     2  0.8648612
 8:  8     2 -0.5993376
 9:  9     2 -0.2663061
10: 10     2  0.0000000