Citing an archival object is not always as straight forward as things you find online or on library shelves.
Check on the bottom of the title page of the Finding Aid to find the Recommended Citation.
This Recommended Citation will be added to whatever item from the collection you use:
Example:
Recommended citation for the Northern Lambda Nord Archives:
Northern Lambda Nord Archives, Lesbian, Gay, Bisexual, and Transgender Collection, Jean Byers Sampson Center for Diversity in Maine, University of Southern Maine Libraries.
in your research you used:
1991 meeting minutes from July.
Your citation would be (in APA):
Northern Lambda Nord, Meeting Minutes for July 1991, (July, 1991). Northern Lambda Nord Archives, Lesbian, Gay, Bisexual, and Transgender Collection, Jean Byers Sampson Center for Diversity in Maine, University of Southern Maine Libraries.
This book is available at all USM libraries in the Reference section. Ask for help at the help desk or circulation desk at the library.