How to automatically create dataframes for each level of a column?

Question

I have the following dataframe (firstweek). The number of rows is very high: 165992. I have 14 different users. I would like to create automatically 14 different dataframes, one for each user. I addition I would like to add in each dataframe a column with the time differences. I am not an expert with R.

> head(firstweek)
                 times        users signal log
14 2014-01-13 00:00:16 00250902DC7A   true  ON
28 2014-01-13 00:00:47 00250902DC7B   true  ON
42 2014-01-13 00:01:18 00250902DC7C   true  ON
56 2014-01-13 00:01:48 00250902DC7E   true  ON
70 2014-01-13 00:02:19 00250902DC7F   true  ON
84 2014-01-13 00:02:50 00250902DC7G   true  ON

> nrow(firstweek)
[1] 165992