sit in
verb
- act as substitute for during someone's temporary absence
- visit in an unofficial capacity
Wiktionary
verb
- To attend a class, educational presentation, meeting, etc. for one time, as a visitor.
- To take part in a group musical performance, especially a jazz session, as a temporary guest.
- To participate in a sit-in protest.
“Over 200 homeless women and children sat-in over night at the state house to protest inadequate state funding for Aid to Families with Dependent Children (AFDC).”