After receiving the invitation to join the Adobe Marketing Cloud, accept the invitation, log in, and accept the End User Licence Agreement (EULA).
This video explains how to set up Target users and administrators.
If you do have an Adobe ID, then your Adobe ID is recognized and you are prompted to sign in.
All Marketing Cloud products appear on the linking page. Click Link Test&Target and enter your Test&Target username and password to access Standard and Classic.
At this point, you do not yet have any groups set up with entitlements for you to link.