I join a multiplayer. Call a portal with L and luckily I'll be around the party. But what I do when the others don't open a portal, I can't find them or a new comer doesn't bother to reach me?
Most of the times I am not able to find the party members on the map when they are distant. I don't know why sometimes they are shown and others not.

If you are in a group you'll see them on the map as green dots if you're in the general area where they are. The area where they are is listed in the party window (press "P") when you hover over the green icon next to their char name IIRC.