Signup experiences can vary wildly based on who your users are and what options you provide them.
One example experience is if you provide email/password as the only option. When a user has a password manager, it's a pretty good experience, but for any users that don't - there's a chance they will forget their password and churn.
Another option you can provide (and PropelAuth supports) is a passwordless magic link. We email your users a link, they click that link, and they are logged in. It's honestly a pretty great experience. The only downside is the user needs to switch to their email, wait for the email to arrive, and then click on it. It's not the end of the world, but it's a little bit of extra friction on top of the "my password manager fills everything in and then I am logged in" experience.
Social Logins / SSO tend to provide the best of both worlds. A user knows they are logged in with Slack, so they click the "Sign in with Slack" button. They get redirected to a page asking if they are ok sharing some basic information like email address with you, and then they are signed up.
PropelAuth manages the entire OAuth flow so all you have to do is enter some basic information and your users can sign in with Slack. These accounts are not special. If you have already integrated, you don't need to change your code at all to work with Slack.
If a user already has an existing account with the same email address that's tied to their Slack account, the accounts will be merged upon sign in.