Swabbies are basically lesser ranking pirates that you gain upon completing quests, scouting, rewards, etc... They support and strengthen pirates. When a Swabbie is designated to a pirate, its status will be added to the pirate’s status as well. In addition, their passive effects will also be shared. So in War Pirates, the company your pirates keep defines and enhances them. The number of Swabbies that gather around a given pirate is limited. But there are ways to increase the number of Swabbies a pirate may have. Like in evolution, wherein matched pirates with the same rarity (star value) as the pirate you want to enhance. You may also use pets (they have rarity also) instead of pirates. The reason why that is important is that it can unlock the previously unaccessible Swabbie slots.
You can access the interface by tapping on the "Swabbie Management" button in the "Pirate Quarter".
To assign a swabbie to your pirate, choose a pirate by tapping the "name of the pirate" button above the pirate avatar, this will then open a selection below of all the pirates that you currently have. Next is choose the swabbie that you want to allocate to your pirate on the right side window.
Tap on the "Set" button and then "OK" to allocate the swabbie to your pirate, and you're done!