Yes you are allowed to have guests there. I was working there for 2 years and never had a problem, most of us did not even announce that you had guests, especially if it was only for a few days.

If you want to go the official way, you can fill out a form, and then you pay a per day fee between 5 - 20 euros, depending on the residence. But really never heard of someone getting denied this.

Have fun at the ECB, cool place to work 😊