Finding and comparing unique values Grouped by Datetime Quarters python

Question

I'm working with an extremely large dataset in a Pandas Dataframe. I'm now trying to understand on a quarterly basis:

• how many UNIQUE sellers have COMMENCED using my product, and
• how many UNIQUE sellers have CEASED using my product.

For example, I have a dataset that looks something like this:

I have a hypothesis on how to proceed, but I'm too terrible at python to see either of the major steps through at this stage. In any case, they are:

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Part 1. Create Lists I've got a hunch that a good place to start might be to create a list of all the UNIQUE SELLERS in a quarter. My 'Month' column is formatted to datetime format, but none of the formulas I've been trying have helped from a 'unique count of seller names' perspective.

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Part 2. Compare Them Once I've got the lists, I then need to compare them to identify growth/attrition rates.

Any help would be very much appreciated. Thank you so much in advance@