Reference: Moving Characters Within Engaged Units, Page 62.

The rules dictate that when a character joins a unit the model is automatically placed in the unit's front rank, displacing a model that is already there.

The exception is if the unit is already engaged in hand-to-hand combat, in which case the character can be placed in any fighting rank (so if the unit is fighting to its side the character can be placed to the side, for example).

This rule works fine when a character model is on the same size base as the unit. However, if the character is on a larger base it is often impractical to swap the character model for a trooper model, as there simply isn't room to do so. This is particularly obvious with characters mounted on large monsters or in chariots, but it is also a common problem with characters mounted on horses, warboars, etc.

The most convenient solution is to place the character at the edge of the unit so that he is touching, but not actually included in its formation, as shown below.

The character has joined the unit and is part of it, but is positioned at the end of the first rank rather than inside the formation itself. This does not spoil the unit's rank bonus, the unit receives the same bonus it would if the character were not there.

To avoid confusion the player must declare that the character has joined the unit, and the model must be positioned so that it is touching the unit base-to-base. It is important to distinguish between models which are part of units, and models which are merely close by. For this reason models which are not part of a unit should be kept a visible distance away - at least 1/2".