I joined a group for CoF that stated in LfG that they were already inside. They told me to join them, I asked how, and they kicked me. Okay, pretty standard for Guild Wars. But now I want to know. The waypoint was contested and the entrance was closed by the stone wall. I was under the impression that if your group was already inside, you didn't need to do the event. But I can't figure out any way to get inside.
You just need to be in the same zone, i.e. in this case Fireheart Rise. When you enter the zone and your party members are already inside, it will ask you if you want to enter (story mode or explorable, depending on what they're doing). You do not need to enter through the gate or be near the waypoint, and it does not matter whether the gate is contested on your server.
Just to round out the answer, if your party is trying to get in, you can always check http://www.gw2state.com/temples.html to see on which servers it is open, and one party member (who has to have completed the story if you want to do explorable mode) can guest and enter there to give the other party members the option of joining.