I will start :) In the beginning I believe in doing things that don't scale.
For example, I would find a group with a niche in my product and start contacting people from the group, offering them some nice perks. BUT this works if you are looking for feedback and not trying to sell them something. Obviously you need to do it in a non-spammy way.
Another idea I have is to contact co-working places and offer them our product as a perk to their members, haven't tried this yet, but sounds promising in my head :D
I think we underestimate referral programs. If they are built correctly, they can bring a lot of benefits and new loyal users to the brand.
Here is a good video on the subject:
https://youtu.be/mQxsyL_BIOY
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