User    App Date
0   A   1   4/23/2019
1   A   2   4/23/2019
2   B   3   4/20/2019
3   B   4   3/15/2019
4   B   5   3/1/2019
5   C   6   3/1/2019
6   C   7   3/1/2019

df_grouped=df.groupby(['Date'])['App'].unique().reset_index()

Date    App
0   3/1/2019    [5 6 7]
1   3/15/2019   [4]
2   4/20/2019   [3]
3   4/23/2019   [1 2]