These search terms from PCORI may give you some ideas for keywords to use in your search. Language used to describe patient and public involvement in academic literature varies widely, so you may need to include many synonyms to do a comprehensive search.
Note that in most databases, an asterisk at the end of a term will search for any words starting with those letters:
participa* will find participation, participatory, participants, etc
For more search techniques you can use in common academic databases, please see this guide.
Many databases have subject headings you can use in your search to find more relevant results. For instance, Ovid Medline (and PubMed) includes the MeSH terms Patient Participation and Community-Based Participatory Research.
For more general information on searching Ovid Medline and CINAHL with subject headings and keywords, please see these tutorials: