Upvote:6
The details of this really depend on the period, the kind of society that fielded that army, the leader(s) who organized the army, and the kind of campaign the army was on. I think it's best to look at the different kind of camp followers, why they were there, and what conclusions can be drawn from that.
- Wives and children of soldiers - this varies a lot.
- Short-term conscripts that were only expected to be in the army for the duration of that year's "campaing season" would not bring their wives along - they needed to stay at home to take care of the soldier's farm, to which he expected to return after the campaign (and would mutiny if he wasn't allowed to). This would apply especially when there was no standing army, such as during the Roman Republic and in medieval Europe.
- Professional soldiers in standing armies were often prohibited from marrying, in part to prevent them from taking their families along. Example: Roman Empire.
- Mercenaries (for whom war was a long-term profession) would very often have wives and children along; any general trying to ban that would have problems hiring mercenaries.
- Personal servants/slaves - whether these were available depends on the organization of the society, but higher ranked or rich soldiers would have these if possible; generals often tried to ban this, but in doing so risked damaging morale (and might even be politically impossible).
- Professional who provided important services to the soldiers: farriers, blacksmiths, cooks, horse-minders, prostitutes, etc. These would indeed typically be approved by an officer and assigned to a specific unit.
- Merchants who sold stuff to the soldiers, or might even be directly involved in supplying the army's regular rations.
As you can probably infer, your scenario of "saboteur blends in with the camp followers of an enemy army in his/her territory" is very difficult for the infitrator:
- Wives, servants and tradespeople were all attached to specific soldiers or units, so strangers would stand out.
- An infiltrator who knows about the structure of the army and the names of its units and officers might be able to bluff their way around by always claiming to belong to a different unit, but that would be very risky - if anyone gets suspicious and verifies those claims, it's game over.
- Posing as a merchant seems like a better option at first, since merchants would pass around different units - but they were also more widely known and a new face would arouse suspicion more easily. Plus, merchants would generally only follow an army in friendly territory since acquiring new wares in enemy territory would be difficult and dangerous.
- Trying to join the army as a new tradesperson or concubine in enemy territory would be extremely dangerous, since enemy civilians were basically free game to rob, rape and murder.