I would walk the team through the Story splitting flowchart. Often someone has an idea how to split it, utilising the full team to split is often better than just trying to split it by yourself. Split during the planning or refinement sessions.
An split here could be something like:
- Login with manual user creation, value is that you can test with real users
- Registration form, value is that the users can pre register
The idea is to get feedback, from users, but also feasibility learnings, e.g. can the team built it. Both splits get possibly user feedback and learnings.
If an 8 is less than 33% of your sprint, maybe don't worry to much about it. Sure 1/6 or smaller stories is ideal, but a 1/3 sized story once in a while is not to bad, just make sure it is your first story of the sprint.