chown confusion

[Huy Ton That]

Okay I'm feeling like an idiot now, if i chowned a directory such that

user 'x' had the ownership of a given directory and was in group 'alpha'

user 'b' needed to add files to the said directory and was in group 'alpha'

now I know usually you do chown :groupname <dir/file> or chown
user:groupname <dir/file> to change ownership however...

I can limit a directory to only a user, but I want to limit it not at a user
level, but at a group level such that all users in a group can write to a
file.

An option to remove ownership perhaps chown -:groupname does this make
sense?