So there will be some options and some limitations in this case.
Adding Contacts
Your best choice will be a contact import so you can add them in bulk, from the Contacts tab you’ll see an import button. Download the template from the template link.
The template will include the correct customer_id but if you do a customer export, you can get all your IDs at once. The contact import will send them to multiple customers at once if you add the right IDs.
Portal Users
You’ll need to decide how you want to handle portal users, and there are two ways. One is where you do not use any portal users. The company will get a link to their customer portal which uses a generated key for them to access sent to their email. It is a bit less secure, as anyone with access to their email can use that code.

They will then use the “Non-Portal User Group” permission set and you can set what they have access to. This will apply to every contact without a portal user- and if you add even one portal user to that company, then everyone needs to have one as it changes how they all log in. All companies will share this permission set.
Something to note, if you ever want to give them end user access to Splashtop, it will require them to have a portal user.
If you’d like to give them a portal user of their own, to bulk assign portal users, head to the Customers tab and select multiple customers with the checkbox icon. Then click Bulk Actions > Manage Portal users. Here you can enable portal users for customers, contacts, or the customer+contacts.
One caveat here is that you cannot select all checkboxes on all pages, just the current page. So if you have a lot of companies to add at once, there may be some manual work. But it will invite all contacts at once. They will then get an email invite to create their username and password.
Assigning Contacts to Assets
This is only manual at this time I am afraid, it would be great if you could set this on a contact import somehow (as well as a portal user invite.) You’ll have to select the contact from the asset detail page. It’s not required to assign a contact, but you do need it if you want to link tickets from the agent contact form to a contact, or if you want a Splashtop end user to access only their own PCs.
There have been some feature requests for company employees to be able to sign up as a contact and a portal user on their own- it’s not something we have the capability of doing right now but we do want to support larger and larger databases so it’s something we’ll have to keep in mind regarding how we want to do that in the future.