Looking for free wordpress membership plugin

https://wordpress.org/plugins/ultimate-member/ is the most solid free one. If you want to do payments, I think you'll have to pay $35 for an extension.

I needed a similar solution, and what I ended up doing is using https://wordpress.org/plugins/popup-maker/ . I didn't need payment, but it allowed me to block content until someone subscribed with an email.

Dude, if you can't "invest" even $20 for a paid plugin you might want to reconsider getting started at all. Free plugins are often not that great anyway, mostly acting as entry points to advertize the paid version. Or they are abandonware and not up to date.
Either way, starting with a low quality plugin will give you negatively skewed results with your initial testing, therefore putting you off the idea altogether.

I think people misunderstand the "MVP" quite often. You see it all the time in this community. MVP doesn't mean doing the absolute minimum because that is basically nothing appealing to customers. Don't forget the VIABLE part of MVP. What is the minimal product I can put to customers that will solve their problem. MVP just means cutting out all the extra bells and whistles and being laser-focussed on your unique selling point almost to the exclusion of all else.

Knocking up a free WordPress website with a couple of free plugins will not teach you anything.

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