You can import contacts anytime by clicking Import Contacts in the left sidebar. From here you can choose to upload a CSV file or connect to an integration to automatically update your data.
If you choose Upload CSV File, you'll see the option to bulk import multiple contacts at once.
You can also add contacts one at a time by clicking + Manually Add Contact in the Import Contacts sidebar.
Note: Any data you import is only available to you and any admins you've added to your account.