Southern flying squirrels (Glaucomys volans) are seasonally gregarious and rely on the thermoregulatory benefits of well-constructed nests and presence of nest mates to survive harsh winter conditions. Prior work has shown that kinship components are important to how winter aggregations form in captivity and that relatedness between individuals is higher than expected in undisturbed populations examined in the field. Over the course of 2 falls, 2006 and 2007, we created related and unrelated groups in a laboratory colony at Wake Forest University. We presented each group type with related and unrelated individuals as intruders. Once formed, aggregations persisted when presented with an intruder squirrel. Squirrels from existing aggregations were found in the same nest box 98% of the time. Factors of relatedness and familiarity were crucial in determining whether the intruder was accepted into an existing aggregation or nested alone. We found that related groups were more tolerant of truly novel (unrelated unfamiliar) intruders and more likely to incorporate them into the aggregation than were unrelated groups. Unrelated groups never accepted unrelated unfamiliar intruders in 2006, whereas some were accepted in 2007 due to increased colony-wide familiarity. Acceptance of intruders was dependent on relatedness. Intruders were more likely to be found in an aggregation when either the existing group was composed of kin or the intruder was related to at least one group member. We also found effects of familiarity of individuals on the choice of nest mates.