It is possible to merge two friendly armies together when neither is at full strength (a full strength army is twenty units in size). Merge by left-clicking on your first army and then right-clicking on the second. The first army will then walk over to the second and the two will merge together. If there are twenty units or more in either army the merge scroll will appear, so you can choose which units to merge into a single force and which to leave behind.
An army that has suffered casualties or attrition requires fresh recruits to replenish troop numbers. If an army contains a general and is on friendly territory, unit replenishment will occur automatically. Without a general, an army can only replenish within a castle town belonging to its own clan. The closer an army is to a province with fresh recruits in it, the faster the rate of replenishment will be. If no replenishment occurs it may be because the army is in an enemy province or an area of attrition.