As a community they are going to be rich, really really rich.
They can walk into any bank vault and walk out with the contents or any penthouse and walk out with the jewlery. They can be the perfect assassins and spies. Their services will be invaluable as long as they remain secret and forgotten. They could walk up to the president in a meeting and sprinkle poison in his coffee without having to change out of their bath robe in the morning because they were sleeping beside his bed and using the presidents bathroom that morning.
What could they offer other supernaturals? Mortal cash. Lots of it and anything which that can buy. They could also offer security. A vampire might like having a bodyguard who can walk up the Slayer and snap an explosive collar around her neck while being ignored or a spy who can sit in on the Vatican's super-secret anti-vampire "lets bless the water reservoirs and pacific ocean" summit.
Communication with mortals
They have difficulty communicating with normal people and using their money since their recordings and writing disappear. I see 2 options here. Recruit a superstitious human willing to believe that the writing that appears in front of them then fades is messages from god. Have him transcribe your instructions into writing that does not fade as it appears and mail it to the appropriate legal firms. Give him "gifts from god" etc to keep him in good condition and cooperative.
Actually you'll probably want a number of people like this but I suspect they wouldn't be hard to find.
Your invisible now have legal entities under their control. They can send written instructions to legal firms representing them who are paid in cash and gold.
Alternatively recruit some of the supernatural beings to act as their agents in exchange for mortal wealth and/or amulets etc from the elderly dead. Again the other beings can interact with the human world on their behalf and take dictation.
If there's a lot of them there's likely to be clans or factions of invisibles who will want to remain secret from each other as well.
Some are going to be richer than others, some might employ different strategies to defend themselves.
Some might defend themselves through obscurity, leading quiet lives while others are going to have their own teams of hired ex-marines hunting any vampires, witches or djinns that come sniffing too close.
If you're richer than Bill gates with a load of puppet corporations under your command you're not going to leave your children to the foster system. They're going to have trust funds for their care as if they were orphaned children of billionaires and since you're invisible you can check in on them and make sure they're being cared for well. Even if their parents are not rich some of these kids are going to feel like they have some kind of guardian angel looking out for them because in reality mom saw the bully make them cry and made sure something awful happened to the bully.
Finding out you're not actually an orphan and that mom and dad were really close by the while time, that the occasional feeling like someone was hugging you when you were sad was actually real the whole time is going to be a very common shared experience among them.
Giving birth might actually be a problem. If none of the staff on the maternity ward can see you they can't really help you. You could die halfway through childbirth while the medical staff ignore you. They're going to need their own staff of invisibles with medical training to give birth.
Actually that brings up another issue. They can learn anything that can be learned from a book but if they want or need any skill/craft that requires training from masters/craftsmen who can interact with them to teach then they're going to have to learn the skill at a young age. Expect bright 11 year olds to find themselves pushed into medical/nursing/ob-gyn training by order of distant guardians or the conditions of some trust fund so that when they finally turn invisible they're already trained enough that they can help the clan/family.
Care for their incapacitated or sick may also be a problem. If you're invisible crossing the street becomes far more dangerous and if you're hit people will keep walking past you ignoring your cries for help. If the clan has any structure for caring for their sick it's going to need to be staffed with their own kind. Secret hospital wards filled with invisible people and invisible staff. A mortal nurse/doctor would have trouble feeding and caring for patients they are incapable of perceiving and/or remembering exist.
They're going to need a significant shared culture simply because they're going to have to rely on each other for a lot of things.
Internal service economy
It doesn't stop at medical care. Any services that invisibles want that require interaction with another attentive human are going to require other invisibles to do it. Need a haircut? good luck with a mortal hairdresser. Want your nails done? the person doing it needs to be able to see your nails. Massage? Ditto. Therapist who can actually hear you and remember what you say? Need an invisible.
So even if a lot of them are spies or assassins others will make a good living providing "human" services internally.
They're likely to have traditions of enhancing their secrecy with fronts and double bluffs. It is after all their most effective defense.
If they have to worry about vampires.Some are going to have front individuals. It's easier to live inside a fortress with guards and anti-vampire defenses if everyone believes the crazy billionaire is a paranoid recluse (but visible mortal) loner rather than the family of 4 who actually live in his 20 room suite with him.
Anyone who does know about them who has need to defend against them will probably have pressure sensitive floor tiles etc to defend against a knife in the back. This includes the invisible's themselves if they have any competing clans or conflicts. Indeed some of the more powerful people in the world might try to recruit them as their own bodyguards. An invisible bodyguard who can see invisible assassins is useful.
Communication with each other
They're likely to have face-to-face meetings or summits or to communicate through any medium that doesn't fade. If Carved stone doesn't fade they can use that, if damage they do to living beings doesn't fade they can communicate through a writing system based on vegetables with scalpels stuck into them in patterns. It's very hard to have all forms of communication fade unless all their effects on the world also fade.