>>53545059I'll give you an answer.
Recently something has changed and 30 seconds after inviting the first 5 people the button reactivates and you can add more. Before that change people would need to quit the lobby and reenter it after 30 secs to be able to do that - no need for that anymore.
To make use of it, I'd recommend LeekDuck - it's like a raiding app but on a website. Look up their guides, how it works. You can set your lobby to have 10 slots.
PokeRaid also has that option I believe, but I've never used that one and since it has a chat I never will anyway. Heard that randoms ask you to invite them instead of the other (correct) way around.
PokeGenie doesn't allow you to get more people in.
In my case I'm still using Genie usually, but I have an alt and plenty of firepower. I invite the Genie shitters, switch to alt, enter and invite some /pgg/ friends, then I switch back to main and invite even more people (whoever is online) from the friendlist.
That being said, the above I applied to Mega Lati@s, Primal raids and now Lugia. Lugia isn't as hard as Lati@s and Primals, but she's popular and there's always going to be interest in raiding her.
Dex filler garbage like genies and utter trash like Mega Slowbro are swiftly forgotten, so people will be very hesitant to waste their remote passes on them.