Hi Anna,
Thanks for reaching out to us; sorry for your experience with the Creative Cloud Express. This shouldn't be happening. I'd like to inform you when an email id is added from the "Invite to" option, an email is sent to the invitee with a link that says "Start Collaborating". Clicking on this link redirects the invitee to the Creative Cloud Express page then the invitee has to click on the bell icon in the top right corner to approve the invitation. Once the user approves it, the page redirects to the project or they can go to the "Shared with You" tab from the "Projects" option in the sidebar to access that project.
Note: You may also ask your colleagues to log in to Adobe Express using the same email id on which the invitation is sent.
Thanks
Manmeet