I starting to data-mine a mobile application, 
and I have a database that looks like this:

```

Database
UserId         Hour         Date
01                18           01.01.2016
01                18           01.01.2016
01                14           02.01.2016
01                14           03.01.2016
02                21           03.01.2016
02                08           05.01.2016
02                08           05.01.2016
03                23           05.01.2016

```

I would like to add a new column to this database that sums the number of different days the user has been using the application,
In this database for example UserId#01 has been on the platform in three different days,

Expected data outcomes like this:

```

Database
UserId         Hour         Date                 NumDates
01                18           01.01.2016        3
01                18           01.01.2016        3
01                14           02.01.2016        3
01                14           03.01.2016        3 
02                21           03.01.2016        2
02                08           05.01.2016        2
02                08           05.01.2016        2
03                23           05.01.2016        1

```

So far I have used this command:

```

Database["NumDates"] % group_by(UserId) %>% summarise(NumDates = length(unique(Date)))

```

But it tells me that that it is creating only 5000 lines (the number of different users in my database) when I need +600,000 (the number of sessions in my database)

If somebody could help me with this, it will be greatly appreciated!