confused by superuser-definition

hello,

i'm rather new to postgresql and am now standing in front of a big problem.

if i want to host my database on any provider out there, i don't now see
any chance to get my own right to insert new users into my db. because:
i'd only be able to do this, when i had the rights to "createuser", but
that  would mean, that i'd have the superuser-rights for the whole db's
in "public" - if there is only this base-schema.

do i understand this right?

if yes: what is a common approach to manage just own users if there is
the situation, that i have a changing amount of them?

btw: why isn't it possible to have (in a future release) a
createuser-right just for single databases and to have one
root/super-user for everydb?

greetings from today very sunny autria
hans