From Naval Action-pedia

A player can move any non-active (not currently selected) ship in their current port to or from the fleet roster.

To add a ship, right-click its selector box in the port "Home" screen and choose the "Add to Fleet" option.

To remove a ship from the Fleet roster, select the "Fleet" menu option and click its "Dismiss/Take" button.

 To return the ship to the port dock, select the "Move to docks" pop-up option.
 The "Dismiss" option will result in forfeiting ownership of the ship without any compensation.

Ships in the fleet roster are selected to accompany your active ship with the white checkbox square(s). Before leaving port, crew must be assigned to each ship up to your total crew limit. This is done through the port "Home" screen with the "Manage Crew" option.

Ships assigned to fleet service ignore any equipped upgrades and cannons and will use the "base" medium cannons for its class.

Fleets can be repaired at sea by using consumable repair kits available from port shops or by crafting.

In battle you can set orders for your accompanying fleet ships, but those orders will be considered more of a suggestion. Your fleet ships will sail and act independently at times.