An external user account's username usually starts EXT_ and they do not normally have a company assigned. You will not be able to assign them to any workspaces.
It's possible to convert an external user account to a full user account if you have BC System Administrator rights. This will allow the user to log in to BC and you will not have to recreate their account to give them full access to the project.
You can upgrade an external user account like this:
- Carry out a user admin search for the external user account. It may be easiest to do this using their email address.
- When you have found their user account in the search results, click on the drop-down arrow beside the user account and choose Upgrade External User account.
- You will be taken to the Upgrade Account page. Give them an appropriate username, and set their company. You don't need to set a password as one will be automatically generated for them. Click "Inform the user by email". Then click the Upgrade User button.
- Assign the user as a member to the appropriate workspaces.