There are clans, and there are group activities. Finding a clan requires going out and looking for one, or getting invited. (I was invited to mine after sailing with them for a couple battles.) With regards to group activities, Post-Captain and Flag Captain missions are, from what I hear (as a Master and Commander), impossible or extremely difficult to run alone. There are also larger AI fleets, found near Cuba and Haiti, that put anything found inside Jamaican waters to shame.