Any 'pro' in l2 has to know about it, but it might be helpful for some people.
First one is gettin into cata/necro withouth being in 7 signs (Just for l2j)
Well the thing is, you need a boxed char/friend/anyone to be into the cata, you /target him and start attacking (cant do it in some catas, i did it only in a couple, for example branded wich has alot of 'forward space' between the gate and where the road turns), so well, you just start attacking/healing or whatever that can get you near the char, and you just relog when you are not moving any longer, and tadah you are into cata.
ANOTHER WAY is, you /target the Rift npc (Dimensional rift manager or something), get into rift (you dont need dimensional thingys) and then teleport back to cata with any npc from the rift,and tadah, you are into cata.
Now in l2j/l2off with geodata.
You need someone to be killing inside the cata, and with the 2nd char (the one not into 7signs) yo just stand up on the top of the room (withouth being into the cata) and you will get the exp of the guy killing inside the cata, its good when you forgot to sign on 7signs or the 2nd char has war and alot of war is inside the cata.
I Hope this wasnt posted, if you dont understand something just tell me, please +post and +karma if it was useful.