Nobody enrolls themselves in a sequence. Either an administrator assigns it, or an automation assigns it for them. There are five ways in, and most organizations end up using two or three of them.
The one to understand first is the form. Somebody applies, and the form either assigns the sequence on its own, or hands you the submission so you can decide after reading it.

Five:
Each is covered below. They can all be in use at the same time, on different forms and different sequences.
There is one link between a form and a sequence, and two places to create it. Set it up wherever you happen to be, and the other side shows the result.
From the sequence. Open the sequence, go to Settings and automations, turn on Auto-assign this sequence via form, and choose your Linked forms. Anybody who submits one of those forms is assigned to this sequence.
From the form. Open the form builder, go to its settings, turn on Auto-assign sequences, and select from the sequences list. Anybody who submits this form is assigned to the sequences you picked.
This is the fastest path from application to onboarding. Nobody waits on an administrator, and the person is standing on their first step within seconds of submitting.
Vome's own advice sits next to the toggle and is worth repeating: auto-assign is only recommended if you do not intend to screen submissions before onboarding somebody. If you want to read an application first, use question 4 instead.
One thing that catches people out: an opportunity's recruitment workflow does not assign a sequence itself. It decides which form the applicant fills in, and it is that form that carries the automation. So put the automation on the form your workflow points at. More on that in Recruitment Workflows on Vome.
Use a question-level automation. Instead of the whole form assigning one sequence, a single answer does it.
Open the form builder, open the question, and go to Question-level automations. Add an automation, set When a user responds with: to the answers that should trigger it, choose Auto-assign sequences as the type, and pick the sequences.
Typical uses:
A question can carry several automations, and one automation can do more than assign a sequence. The same panel also assigns opportunities, profile tags and sites, notifies administrators or the user, and sets the submission status. Anything you build here is listed under Admin-created automations, separately from the turnkey switches Vome provides.
This is the path most organizations actually want, because it puts a human decision between applying and onboarding.
Open Forms, open the form, and open the submission. Read the answers. If this is somebody you want to move forward, open the actions menu on the submission and, under Assign, choose Assign sequences. Pick the sequence and save.
Two things worth knowing:
From the Database, which is where most assignments that did not start with a form happen.
Open the profile and use its actions, or select one or more rows and use Take multiple actions, then Assign sequences. Any active profile can be put into any published sequence, at any point in their time with you.
The same Assign sequences action turns up in a lot of other places, and it behaves identically in all of them:
Wherever you assign from, you can tick Notify users about this sequence assignment and add an optional message. Leave it unticked and people are enrolled quietly. A sequence set to be visible to administrators only notifies nobody either way.

Yes. On the sequence's Settings and automations, Auto-assign sequences enrolls somebody into the sequences you choose the moment their profile is marked as finished. This is how general onboarding chains into role-specific training without anybody assigning anything by hand.
The same screen can approve them on opportunities, add them to your database, and apply profile tags and sites at that same moment. Full detail is in What are sequence automations and how do I use them?
No. A sequence is a process, and onboarding is only its most common use.
Any user can be assigned to any sequence at any time, whether they joined last week or five years ago. Sequences get built for:
Because one person can be in several sequences at once, and because a shared requirement is completed only once no matter how many sequences it appears in, putting a long-standing user into a new sequence is normal rather than a workaround.
Three usual reasons: