You can put a candidate into a talent pool from a few places. A candidate can belong to several pools at once.
From the candidate profile
Two ways, both in the profile's action sidebar:
Click Assign, switch to Talent pools in the dialog, pick a pool, and confirm.
On the Talent pools card, use its add control to attach the candidate to a pool. The card also lists the pools they're already in — the picker only offers pools they aren't in yet.
While adding a candidate
When you add a new candidate (manually, by CV, or by CSV), the Destination step lets you drop them into a talent pool right away — see How to Add a New Candidate.
In bulk from the CRM
On the Candidates list, select several people and use the bulk bar to add them all to a pool at once.