User Registration Feature to Create AND/OR allow existing users to continue

+1 for this with a different use case.

Using a form for registrations for an annual event and needing people to sign-up & pay each year.

Is there a way during the submit validation to check if email exists and if so not execute the user registration connected form?

A) What you describe is possible, but separates the registration for site from event. That's not something which fits well for this case which is closer to renewing annual payment. Along those lines, and lowering barriers to engagement, if the user doesn't remember their login (or that they even have one) instead of them having to go through the lost / reset password process, I'd like to be able to accept their event registration and pass on details in the follow-up message to remind them.

B) I could envision this as a user registration form settings option to control whether email validation should trigger account block. OR perhaps a hook which allows override through code whether matched email sends an error to user or aborts the user creation.

C) Current stop-gap has been to add a field (yes/no radio) asking if they have registered in a previous year and uses the registration form enhance to control whether to create accounts or not. It does leave a window of opportunity to have registrants think they have an account but do not and not get one created - but I can do a manual check for the volume of registrants involved and create as needed.

+1 for this as well.

We are running a network of radio station sites via multisite and are using the the Registration Add On to allow people to register for that one particular radio station site (this bypasses Multisite wanting to push everyone to the root website to register). The problem is if they want to register on a 2nd site, they are already in the global database (so they get a duplicate email error) but are not actually members of that site.

The solution suggested above would help because it would add that existing user of Site 1 as a member of Site 2 (while ultimately recording only 1 user in the global database).

That said, is there an easier way around this?

We also have this issue. We use the user registration feature as a way to unlock additional content on our site. To view the content on a later visit (i.e. log in) we only ask for their email address (and we only prompt for their email address if they click to view this additional content). We have a form that allows users to register with our site to view this additional content, as well as make other requests.

We want people to be able to submit this form with as little difficulty as possible. The issue for us is if they fill it out to register, then later come back to fill it out again for other requests, and happen to select the option to register again, then they see an error message that blocks them from submitting the form.

We need to be able to ignore the user registration option at this point. As a work-around, we could put a message on the form to the effect of "If you have already registered with our site, then make sure you do not select the option to register again". But we will potentially lose a lot of our users in this process.