First thing to do in multiplayer IC?

Probably, food + the same basic stuff you'd make in normal minecraft.

I'd make a treetap really soon and start collecting sticky resin + saplings and seeds/etc of things you like.

Then you gear up for the journey to the center of the earth (for resources).

It's all in the FAQ somewhere...

However you'll end up cooking some treesap (not all, just enough) to make a generator + macerator, then extractor, and finally your initial bootstrapping is complete at that point. After that personal preference dictates more than requirements. However I'd personally go for a mining kit at that point. If you only want to make one manual trip to bedrock I suggest totaling up what it will take to build your first basic miner and making sure you get enough to do that.

Be sure to make a personal safe as soon as you can - they store as much as a double chest in half the space, and others can't steal your stuff.

Beg for resources? Or at least borrow something from someone who can afford it and re-pay double once you are set up.
That's what most of us do when we arrive late to the party. The true multiplayer experience can only be had on a new freshly generated world. (Unless the server is PvP, in which case you should join an alliance or form one)

Hack into the local power grid and get enough EUs to run a stolen UU Matter Gen. Use this to get the basic resources to construct a Miner and a Compressor. In the meantime hunt/farm creepers and convert all UU-Matter to gunpowder. Construct a giant nuke block (at least 10x10 chunks, top to bottom) and detonate it all. Get banned.

j/k

Well if you get late to a party use recyclers of other ppl to get tons of scrap boxes. Not like they would really care if you use them but it has some good drops.

Quote from DrSNAFU

What I call Phase 1 consists of 1 generator, 1 macerator, 1 electro-furnace, and 1 rechargeable battery. That’s bare-bones for a mining setup. That way you’ll start macerating your ores and smelting them down. You’d have to use tree taps and the furnace to (inefficiently) extract rubber until you have enough resources to add an Extractor to the setup, along with wiring and probably a bat-box in Phase 2, solar panels are added around that time too.

Use insulated copper wires instead of a handheld battery...
Also redstone is worth enough power to Macerate an ore so I normally use it to make the dust for a generator.