userID  browser         slot    n_clicks    n_queries   n_nonclk_queries
0   1       Browser #2      exp         23          32      19
1   3       Browser #4      exp         3           4       2
2   5       Browser #4      exp         29          35      16
3   6       Browser #4      control     12          6       0
4   7       Browser #4      exp         54          68      30

In [221]: df
Out[221]:
   userID      browser     slot  n_clicks  n_queries  n_nonclk_queries
0       1   Browser #2      exp        23         32                19
1       1  Browser #33      exp       100        100               100
2       3   Browser #4      exp         3          4                 2
3       5   Browser #4      exp        29         35                16
4       6   Browser #4  control        12          6                 0

In [222]: df.groupby('userID', as_index=False).sum()
Out[222]:
   userID  n_clicks  n_queries  n_nonclk_queries
0       1       123        132               119
1       3         3          4                 2
2       5        29         35                16
3       6        12          6                 0