Two ways in
With a code. Ask the adviser for the 7-character class code. Join with a code → type it → pick the subject you teach → the roster is already there. This is the best path: nothing to type, and your grades reach the adviser's report cards on their own.
A code can also belong to a single gradebook rather than a whole class. That kind adds you to that subject as a co-teacher, beside the teacher who owns it.
Standalone. My subjects → New gradebook if no adviser is on SchoolAide Lite yet. You give the school, year, grading periods, grade level, section name and subject, then bring your own learner list. When the adviser joins and creates the section, you both get a prompt to link. Linking merges only the learner list — every activity and score you encoded stays exactly as it is.
If the code is not accepted
"No class or subject has that code" means those characters match nothing live. Codes contain no 0, O, 1 or I, so a letter that looks like O is a Q or a D, and a 1 is a 7 or a J. Check with whoever shared it. An archived class no longer answers to its code.
If you were invited by email
An invitation link opens the join page with the code already filled in and drops you straight into the class record for that subject. Invitations expire — if yours has, ask the adviser to resend rather than editing the link.
What you can see
Your own subjects. In a section someone else advises, the other subjects, the roster details, attendance and the release tools stay with the adviser. Where that boundary applies, the screen says so plainly instead of showing you a blank page.