If you use open remote to access the database, you should gain access to your database and the fact that you have opened the database as a FileMaker client won't keep your "up to 5 IWP users" from accessing the database at the same time.
BUT making design changes to a hosted database while it is in use is very risky. Simply opening Manage | Database and examining a field's auto-Enter calculation locks all other users out of modifying any record in the entire table. If a script is run by another user that attempts to modify records in that same table, the results could be catastrophic.
Any network glitches while committing a design change can damage your file.
And any sudden change in database function brought about your your changes--possibly in the middle of a user's interactions with the database could "catch them between stools" leading to any number of problems.
Small adjustments to layouts and scripts are not as dangerous--I often make those adjustments myself, but there is still a risk of trouble.