Group your contacts into audiences, and store your own details on each contact. Open Contacts in the dashboard and you'll find Audiences and Fields tabs.
An audience is either a list you curate or one a rule keeps current, matching on things like tag, status, and when the contact was added. Fields hold whatever else you need to know about a contact.
Prefer code? The same features are in the API and SDK:
await samva.contactGroups.create({ name: "Beta testers" });