Holocnemus pluchei spiders (Family Pholcidae) facultatively live in groups: sometimes they live alone and sometimes they share webs. In the field groups vary in size and composition and include spiders of all ages and either sex. Group membership is flexible and individuals move frequently among groups. To understand group formation and maintenance it is necessary to understand the costs of group membership. We used focal animal sampling to investigate the cost of group living for spiders of different ages across a range of group sizes. Both spider age and group size affected the costs incurred by group-living spiders. There was no variation among groups of different sizes in the percentage of time focal small or large spiders spent in costly behaviors (moving, web maintenance, bouncing or interactions with conspecifics), but medium-sized spiders spent more time engaged in costly behaviors with increasing group size. Medium and large spiders also had more interactions with greater numbers of different conspecifics when they were in groups larger than three, whereas small spiders interacted rarely with conspecifics regardless of group size. These results suggest that there are significant ontogenetic shifts in the costs of group living in H. pluchei.