gasta

This is the main reason for NOT using Members Area. Squarespace need to communicate a roadmap here, otherwise this product is not interesting.

This is the most frustrating limitation of Member Areas (MA), and there's nothing you can do about it. No approval mechanism exists and it is not possible to add one. You can try to workaround the issue by hiding links and password protecting signup pages, but this is known as "security through obscurity" (trying to hide the security flaws). It is poor practice and won't provide any real security for your MA.
In 2021, I published the list of Member Area Limitations that I had shared with Squarespace, and this was top of the list. 
Squarespace don't routinely monitor this forum for feature requests or customer questions so, if this feature is important to your business, I recommend you contact Squarespace Customer Care to log your concern about the delay for this feature. The more they hear from customers who are affected by missing features, the easier it is for them to prioritise them.
If you are a Circle member, you can vote here: Require approval of members to join Member Sites.
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This is the main reason for NOT using Members Area. Squarespace need to communicate a roadmap here, otherwise this product is not interesting.

This is the main reason for NOT using Members Area. Squarespace need to communicate a roadmap here, otherwise this product is not interesting.