Typically, solely at the discretion of the service provider, there is no charge for you to be a User within a service provider's Portal.

Being invited to 3 or more Workspaces within a Portal constitutes being a 'premium User', which is a paid-seat in that Portal.
Your service provider, at their discretion, may bill you for 'premium User' charges.

Depending on your specific use-case, you can discuss options with your service provider that don't result in you being a 'premium User'.

These options, again: offered at the discretion of the service provider, are:
Reduce the number of Workspace Invites to 2 or less, by making use of sub-folders within a single Workspace, rather than having access to multiple Workspaces.
You could use a different login email (as username) per Workspace (which you have to keep a record of/keep up with) should the information per each of your entities need to be kept strictly separated. 

If the correct fit for you is to be a 'premium User' at your service provider's Portal - but yet you are being billed for multiple 'premium Users':
You can discuss with your service provider whether utilizing a shared credential among your 'premium Users' is viable, to keep fees down (it will depend on the security needs of your specific use-case at the service provider's Portal).

All billing questions must be communicated to your service provider, whom owns the Portal/Workspace. DO NOT contact DollarADay Websites about your service provider's billings.