You will need to activate an account. If you must exercise your stock options with a captive broker or a company-approved broker, you will be required to establish an account with that brokerage firm. Similarly, your company may choose a firm or transfer agent for vested restricted stock/RSUs and for purchases through an ESPP.
However, you can transfer the shares to another brokerage firm after this initial event. If you are planning to sell the shares immediately or very soon after option exercise, restricted stock/RSU vesting, or ESPP purchase, it will be much easier and efficient to use the company's designated broker.
Alert: You will be required to set up an account with the broker or transfer agent that the company uses, at least for the initial transfer of shares from the company to you, whether in an option exercise, upon restricted stock vesting, or at ESPP purchase. Failing to set up this account before the date needed will delay your ability to sell the shares. In addition, when you plan to hold the shares, you should check any standing order or default rule that will apply for which stock in your account to sell first (see a related FAQ).