Best plugin for user registration to sub-site

Before posting a HELP! message I did a little searching and discovered that user registration defaults to “Super site” or whatever. Several plugins appear to be available.

My problem: I just activated a blog on a heavily trafficked site, and the registrations are coming in, with users assuming they are registering for the main site (https://ssnet.org). However, they don’t show up as users for the site — only in the Network menu. I need an easy way to have them registered to the actual site, without any more action on their part and, preferably very little action on my part. (Don’t relish manually copying & pasting thousands of usernames into the actual blog site. My hubby’s already resentful of the time I spend on this.)

There must be a “best” plugin for this.

I’d really appreciate a heads-up for the best plugin.

Thanks.
Inge