Hi Lingtao Pan.
therefor open source version won't be usable for more than a single user install or just a bunch of friends. And even when used amongst friends, it's sometimes required, that they don't see each other. When you'll get married, you'll understand. ;-)
I would understand, that advanced features, like working with MS Office files directly via web browser or LDAP integration could be payable, but to have all users on your cloud visible to all other users by default should, be a part of open source release available to everyone.
As this basic functionality is needed, i wonder how much will this "enterprise" edition cost.
Will the charges be reoccurring for each new release or on yearly basis?
Will you charge per user?
For people using this in business environment, cost planing is very important and if they are hit down the road by costs that are not be expected, they won't be very happy.
I personally don't mind paying a small one time fee or give a donation as i did in the past to open source projects which i use in my daily life, even thou i'm already paying by advertising your services (seafile.cc) to each of my friends/users by giving them access to my seafile install.
What i could see happen, if this basic functionality will be payable is, that someone (might be me or one of my friends) will code this group separation and while doing it, will prepare a release without your advertisements (like CentOS did from RedHat) and people will start using that fork, because it will contain basic functionality, like group separation and will contain _no_ advertisements.
As i've been long in this post already, let me sum it up and finish.
1. Seafile is a great product, especially when compared to owncloud. And your support is awesome for now.
2. Basic functionalities like option to remain private to other cloud users outside your group should always be free,because it's basic functionality.
3. It's perfectly acceptable that advanced functionalities needed for big installs or enterprise needs are payable. Functionalities like LDAP integration, editing MS Office/ Libre Office files, .. etc could be payable. When deciding what should be payable, try and ask yourself what would a big enterprise install need ('cause they have the money to spend), while small installs won't.
Hopefully you will consider my opinion.