For the login, you would create a login form and based on who they are, direct them to the appropriate landing page. This can be tricky to update if you're hardcoding all the partner companies into your script (don't do that). Instead, for each company, create a user. Then, based on the users role, they can access pages that they are allowed to manage. You might find the Page Edit Per User module handy http://modules.processwire.com/modules/page-edit-per-user/ Or the Page Edit Per Role module http://modules.processwire.com/modules/page-edit-field-permission/ .
I am actually working on a front end admin for users of a company intranet and that's along the lines of how I do it.
Well then, ensure that the Language Support - Fields Module is installed. Then, go to Setup -> Fields -> Body and change the type from Textarea to Textarea Language. That should make the field so you can edit it, but without TinyMCE working. I don't have the most experience with the Language Support stuff so maybe someone else can chime in.
MindFull's post in Project progression to portal was marked as the answer