It is possible to invite five people, leave, rejoin again and invite another five hitting the hard limit of 10 people invited, but 9 times out of 10 raids will have the five people invited and that is it, so six people max in a room.
I'm not the greatest for advice as I don't raid a huge lot - some others will be better at this - but I don't really stack-up my stardust too much.
You can generate more stardust during certain Spotlight Hours (next Tuesday, for example). See below for more info on Spotlight Hours and their bonuses:
https://pokemongo.fandom.com/wiki/Pokémon_Spotlight_Hour
You can get stuff including stardust by battling each day in the Go Battle League. Up to 25 battles per day, here are my stats - and that is just from over a season:
You can get decent Pokemon as encounter rewards from it (3 wins) with some decent CP - I think I only used stardust on Rapidash and Cloyster for GBL.
I've amassed 152 rare candies primarily from battle rewards (4 wins in a set), and some from raids. It is suggested that you use rare candy on the legendary/mythical Pokemon (1 rare candy => 1 Pokemon candy transfer rate) with them being the rarest. I'm planning on using mine on my Rayquaza once I have a decent Pokemon to replace it in Ultra League.
Here are my highest rated Pokemon
Others here will have sets of five or six Pokemon like Machamp or Rhyperior for raids (I think there were community days for them a while back). Mine aren't optimised but I do okay on the whole in raids.
In the wild at the moment I'm finding quite a few evolved Pokemon like Lairon, which will be higher CP so they'll help. And on Sunday there's an event for Beldum which can be evolved into Metagross which is good for a few raids too.