You have to enable it globally (on the cogwheel go to Products and then under Jira Service Management click on Configuration -> Customer permissions). Once it's enabled globally, go back to your project settings and you can allow anyone on the web access without logging in, or you can keep it so that they have to be logged in but at least now they will be able to create their own account without an admin inviting them.
adding to what Kristin said here is some documentation about the login-free portal you could use.
Note that this is not a good option for every company - some admins prefer to have things locked down a bit more strictly like it would be using the login-free portal, as Kristian already outlined.
The takeaway is that Jira can be (also in this particular part) configured flexibly - please also see here for further documentation on what we discussed here:
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