Hi, Phoebe -
You can set member group permissions to protect entries so authors can touch only their own content:
Can view weblog entries authored by others
Determines whether a member of the group is allowed to view weblog entries that have been authored by someone else. Note: One entries within weblogs assigned to the user (see below) apply.
Can delete their own weblog entries
Determines whether a member of the group is allowed to delete their own weblog entries. Note: One entries within weblogs assigned to the user (see below) apply.
Can edit entries authored by others
Determines whether a member of the group is allowed to edit weblog entries that have been authored by someone else. If this is set to yes then members of this group will also be able to view weblog entries authored by others regardless of that permission’s setting. Note: One entries within weblogs assigned to the user (see below) apply.
Can delete weblog entries authored by others
Determines whether a member of the group is allowed to delete weblog entries that have been authored by someone else. Note: One entries within weblogs assigned to the user (see below) apply.
So yes, when creating a post, the author would need to be set to the member you want able to edit only that post. But after that, the rest is taken care of by EE.