When looking at your Trade Partner list (Contacts > Trade Partners), if there is a check in the checkbox under the web access column, it means that the trade partner has registered with the invite email that the system sent to them and now they have a password and can log in.
If it says "Invite Sent" then it means they have been sent an email inviting them to CoConstruct but they have not yet registered. That means that they have not yet logged in to the system.
Use the "Resend" link to send out a second invitation, since they may have lost or discarded the first welcome email.
If you have resent the web access welcome email and they still have not registered, you may want to personally contact them after a period of time to explain what the registration email is for and that they need to use that email to secure their login email address and password so that they can have access to your projects.
If your trade partner needs a password reset sent to them, they can request one by using the "Forgot Password" link on the main login page. You can always enter their email in the Forgot Password form to reset this for your trade partners.