Can one company use media of another of the same group?
Well look, the issue that one company uses the resources of another of the same group to get into a public tender ... that is not so weird, but it is not that it can be done so, without more. There are rules, laws, and quite small print there, because the authorities like to have everything under control - nobody wants them to be cooked a “ghost” that cannot fulfill the contract.
In the legal world, this of using the resources of another (either of the group or outside) is usually called "subcontracting" or "pulling the capacity of third parties." And yes, in most countries, this is allowed, but eye, with clear conditions. For example, in the European Union, the 2014/24/24/that allows it, it does not matter if companies are cousins, sisters or only known. The trick is that the company that goes to the tender has to show the public body that, if it wins, you will really have those media by hand. Nothing "we'll see." Many times they ask for a role signed by the other company saying, "hey, if you win, I leave my resources."
If we talk about business groups ... Well, it is normal for one company to throw from another of the group to add muscle. But, again: we must prove that this access to resources is real, sure and is not going to fall in the middle of the contract. It is not enough to say "we are from the same group, quiet."
Another thing: the company that relies on another has to meet the selection criteria, it is not worth presenting for presenting. If you are going to use your “sister” media, make sure that company meets the technical requirements or what the specifications ask for. If not, you stay out.
And be careful, sometimes the authorities put limits or directly prohibit this issue of subcontracting or depending on third parties, to ensure that the company signed by the contract can realize. So, before launching, you have to read the bidding documents with magnifying glass and, if necessary, ask a lawyer who knows the subject. Better prevent than regret.
Summarizing: Yes, you can use the media of another group for a public tender, but you have to follow the rules of the game, demonstrate that you have real access to those resources, comply with the criteria that ask for and not skip any limitation they impose on subcontracting. Nothing to go crazy, then the scares come.