Each school community has a unique registration link. If you are a member of more than one school using PTAsocial, e.g. infants and juniors, you will need to register separately for each (unless the PTA is jointly run). This is to ensure that each community is secure and only has valid members joining it, and that targeted news can be shared with each set of members.
Ask your PTA for the link to register. They should display the link on the school or PTA website ideally. Once you are a member of more than once school community, you can switch between your communities using the "Change community" button.
When you access the app from a smaller device screen, e.g. mobile, you can switch using the "Change community" icon, the up and down arrows.