References
My Father’s House
October 25, 1850 Fitchburg Sentinel

Commentary
The visit of most of these abolitionists is known only from My Father’s House, a monograph published in 1915 by Benjamin Snow’s daughter Martha Snow. Since she was writing 65+ years after the fact when she was over 80 years old, not all of her monograph is considered accurate. She was undoubtedly in Snow’s house during the abolition period but the details of who visited and when are very hard to verify. Frederick Douglas is mentioned as attending an abolition meeting in the 10/25/1850 Fitchburg Sentinel. The Sentinel also published an ad for a visit by Solomon Northrup author of “12 Years A Slave” (more on him here). Reference to other visitors in the Sentinel or elsewhere have not been verified, yet.

Suggestions for further research
Continue searching in Fitchburg Sentinel for references to these people visiting Fitchburg. Also, start with their owns writings (e.g. look for diaries or memoirs of The Grimke’s where they describe their travels).