Change Header (Logo) Based on Login

If WP Members (or another plugin like User Access Manager) provides conditional tags for specifying the current user’s “group” then you could use that to switch out the logo. One of those might also give you a way to insert a class into the <body> tag that you could use to switch out the logo via CSS.

OR, if you just have one user per company, you could add the current user’s username to the body_class() and use that.