If you share a folder with a free DriveHQ member, the member can access your folder in the special folder: \DriveHQShare\Your_UserName\Share_Name
The member will use your account's storage space and download bytes. So it does not matter if the member has enough storage space or not. I checked your account, everything seems to be fine. If the other member is limited, it probably is due to other reason. Please ask the user to email a screenshot to DriveHQ support directly.
We recommend creating guest accounts for sharing folders with external clients. This way, your clients don't need to sign up on DriveHQ themselves. It is more flexible and more manageable on your end. Because a guest account belongs to your group, you can easily edit, disable, delete or re-use it for another client.
You mentioned "Our client, using a free DriveHQ account", but then you also mentioned that "since they are using our credentials", it is a little confusing. If a client logs in with your login credentials, then our system will treat the client just like yourself. Please email us what limit you encountered.
We have a lot of tutorial videos. Please watch some of them.