I am only a user, like you. I have no connection to Apple.
A CLEAN reinstall of El Capitan is likely to help.
Notice the all-caps?
In the last few days I have seen users reinstalling EC over and over in the hopes of fixing problems - only to reinstall their problems because they migrate their old system over. This is clearly a case where something from your user account, coming back from the previous system, is conflicting with something in EC.
That is why a clean guest user shows no such problem. Sharing does work in EC, for most users.
The really tough thing is to figure out what exactly causes this.
I see to escape routes:
1) Create a new user account, and use that instead. Some difficulties may arise in copying documents, mail, photo libraries.
2) [This can be done in a second partition or an external drive for the moment, in order to keep your present system available. ]
a) Clean install EC. Create a new account with the same short name as your present account (this is not mandatory, byt may help in step c) below)
b) Run it for a bit in this clean configuration, to check that it is working
c) Migrate ONLY your user account, not applications or settings. (if you named it exactly the same, you will be offered the option to merge those accounts; do so). Fingers crossed that everything will still be ok.