To invite other colleagues to collaborate with you:
Click on "Colleagues" in the left-hand menu. There you will find a list of your colleagues.
Behind each of your colleagues' names, you can see if they already have a teacher's account or not. If they don't yet have an account, you can click on the three little buttons ( "..." ) on the right-hand-side corner of your colleagues' names and directly invite them to create an account.
Another way to invite your colleagues to collaborate with you is to directly click on the blue "Invite Colleagues" button in the right corner of the "Colleagues" page.