A unit automatically starts in formation with his group leader, unless you select "none" in his Special field. However, even then, he usually move into position as soon as the mission starts.
There are various ways of making him stay where he is, and which commands are most appropriate depend on exactly what you want. Look up the following commands in the online comref, which has helpful Comments
doStop
disableAI
setUnitPos
Have a look in the Ed Depot for scripts too, I have a feeling there might be something helpful there.
In buildings there are numbered positions: it is possible to put a unit in a position though I can't remember exactly how. If there is only one unit in the group you can give him a waypoint exactly on the house and get new options in the waypoint.