Depends on the group of people. If they like the Warhammer setting, or grim fantasy in general, if they like other deckbuilding / CCG / LCG games, if they are comfortable investing in a game before they know if they like it …
In general, I would simply regularly bring the game to your game nights or club meetings or whatever you have, and gently encourage people to try it. If they like it, let them keep playing with your cards, while introducing them to the idea of deckbuilding, until they express an interest in getting their own core set (or more). If the financial aspect is an issue and you are comfortable not all having access to all factions, a group can split the core set and the expansions to save money.
As for which expansions are good - all of them. As a beginner, you will get the most value out of the deluxe expansions though, starting with Assault on Ulthuan.
There are no official multiplayer rules, but a whole lot of fan-made rules which can be found either here or in the BoardGameGeek Invasion forums. They all have different strengths and weaknesses, but everyone I know who's interested in multiplayer Invasion seems to have found a ruleset that works for them, so you shouldn't have problems there.