You can add contacts to an account in either of two ways: 1) Start on the account screen and attach existing or create new contacts from there or 2) start on the contact screen and link to an account from that direction.
Keep in mind that individual contacts may only be associated with one account at a time. However, accounts may have unlimited contacts attached to them.
Accounts feature primary and secondary contacts. These can be the two contacts you work with most often. However, you can add as many additional contacts as you like to the account.