All of your characters on the same account are automatically full owners.
If you have more than one account, to add the other account's characters as co-owners, you stand them within 5 steps of the sign, log out, log back in on the owning account, and go into the house sign's security section. Click the add co-owner button, and target the other character (who should still be standing there, waiting to time out, after being logged out in a place they are not friended or co-owned to).
Repeat as necessary.
This is for 2D - the procedure for adding co-owners is a bit different in KR (but the main account's characters are still always considered owners if the house was built by their account)