I am a graduate of the Islamic University of Madinah. Some brothers invited me to give the Friday Kutbah in a Musalla (praying place) in their company. I found that entering the company is restricted to the staff only due to security reasons and the nearest mosque is 10-15 minutes away by foot. I am not sure whether the company allows its employees to go to the mosque or not. The number of worshippers inside the Musalla is 10 in the summer and 35 in winter. Even if I refuse to give the Khutbah, they keep on offering Jumu`ah prayer in that small Musalla.
The basic ruling is that they should go to the mosque to offer Jumu`ah prayer and not pray it in the Musalla, as long as the mosque is near. It is not permissible to pray in this Musalla unless there are reasons like the ones mentioned in the question. If the company has a strict policy that prevents its employees from going to the mosque, they can pray in this small place. The Hanafi school of jurisprudence stated the permissibility of offering Jumu`ah prayer in two or three mosques at the same time. Yet, if the company does not ban its employees from going to the mosque, there is no reason to stay in the small praying place. They should go to the big mosque and offer Jumu`ah there. This is the opinion of the majority of scholars (may Allah be merciful with them all): holding more than one congregational prayer not due to narrowness or farness of the mosque is not permissible.