To invite other colleagues to collaborate with you:

Click on "Colleagues" on 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 teachers account or not. In case 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. 

An other way to invite your colleagues to collaborate with you is to directly klick on the blue "Invite Colleagues" button in the right corner of the "Colleagues" page.