There is a simple slogan that we can use when trying to approach groups in silos: attend, befriend, and defend. The idea works like this:
The first thing that we need to do is make ourselves available to the group as much as we can. We need to show up for the team meetings or events. In lean terms, you might think of it as going to the gemba. You need to be where the action is. This may involve getting yourself invited to meetings, usually the most boring and uninteresting meetings. You take anything you can get. Then you have to be there all the time, religiously. They have to come to believe that you are genuinely interested and part of their process.
Once you have the attendance part down, then it is time to make a few friends. You need to be that smiling face in the room that is ready and willing to help with even the most inconsequential problems. In fact, beginning with the small problems is probably the best way to start. We need to be there to offer help, and then when the offer is accepted, deliver. This is really all about building up trust with the new group. It is just trust with only you, but you have to start somewhere. You do everything you can to build that relationship. Go out for a beer, Invite them to your team meetings, join the softball team. Do whatever it takes.
Finally, it is inevitable that they are facing challenges; this is where you can really make a difference. When the opportunity arises, be there to defend the group. Make sure that you radiate the fact that you trust and support the work the group does. Not only does this serve to cement the trust you are trying to build within the group, but it also informs the rest of the organization that you are working together and share common goals.