If you have multiple users connecting to FTP via a single FTP account at the same time then you may find that you regularly receive a message stating that you've exceeded the maximum number of FTP connections (3).
Each FTP account does have a simultaneous connection limit of three - so the best way to resolve this is to simply create an FTP account for each individual user.
You can create FTP accounts for a domain through the Cpanel control panel -> FTP Accounts.
Additional FTP accounts can be restricted to specific directories, so you can limit some users to only have access to a certain area of the site rather than the entire thing if necessary. Particularly useful if you are allowing them to run their own minisite under an existing site, for example.