Min Date Per Group

[Okrasee]

I want to calculate the earliest(min) date per group in my explore.
I applied formula: add_days(min(diff_days(now(),${my_table.date})),now()) but it returns the earliest date among all rows as in for each row the earliest date is the same despite of different groups.
What I want to achieve is:

Group                Date
A                    2020-01-09
B                    2019-08-01
C                    2016-10-05

Rather than:

Group                Date
A                    2016-10-05
B                    2016-10-05
C                    2016-10-05

Would this be possible? Making changes in lookml is also welcomed but adding custom calculation would be ideal. Thanks in advance.