Using cumsum with 'group by' ?

Hi,

First post here. Grateful for any help you can give. I have data which looks
like this:

id    time    x
1   12:01    5
1   12:02   14
1   12:03   6
1   12:04   3
2   12:01   98
2   12:02   23
2   12:03   1
2   12:04   4
3   12:01   5
3   12:02   65
3   12:03   23
3   12:04   23

But I want to add a column which is the cumulative sum of X, but only by id.
I've used cumsum before, but not in this way. So the result should be
something like:

id       time      x        cumsum
1      12:01       5        5
1      12:02      14      19 
1      12:03      6         25
1      12:04      3         28
2      12:01      98       98  
2      12:02      23       121
2      12:03      1         122
2      12:04      4         126
3      12:01      5          5
3      12:02      65       70
3      12:03      23       93 
3      12:04      23       116

Any ideas please?