Songtrust only represents you and we will help you get your share of the royalties owed. Co-writers will need to create their own Songtrust account to collect royalties or find another way to collect their share.
You can and should still register them as co-writers by adding them as outside writers on your account. This can be done from the songwriter's page on your account, as shown below.
If you are unsure of all of your co-writers' information, you can read this article to find out what to do.