Payment for publication after graduation
Who should pay for the publication? In most cases, the money comes from the research project, or main research institution, i.e. in most cases the funds should be appropriated by the team leader or research supervisor. This typically means that the senior author is responsible for paying the article publishing fee.

If the research project was a collaborative effort with no specific funding, another option is indeed to ask all collaborating institutions (not individuals) to share the cost. This would include all institutions, not individuals, and probably only staff members — grad students or post-docs typically do not have their own independent funding.

This should have been clarified before the paper was submitted to publication. This is the best practice whenever submitting a paper to a journal that requires paiement (e.g., gold open access venue), to avoid surprises and disappointment from all collaborators.

In no case should you use you own personal money to pay for this publication! This is a professional cost, and it should be paid from institutional funds.