Do you know how global fields work in a hosted database? IF not see this article: http://help.filemaker.com/app/answers/detail/a_id/5895/kw/5895
This is probably the biggest difference in behavior between a single user and multi-user database.
Do you know about "edit locking" records? IF user A starts editing a record, all other users are locked out of making any changes to that record until user A commits the record. This can be a particularly "sticky" issue if you have a script that needs to update a large set of records and another user has locked one of the records--such "housekeeping" scripts are best performed after hourse when there are no users logged in.
If needed, "user preferences" can be logged into a table that a script checks when the file is opened. Typically a field in this table records the user's account name so that the script can use it to find the correct record of users preferences.