A battlegroup with T2 or T3 units can request new units by clicking the group, then the unit buttons on bottom. Factories will build the units, which will automatically be added to the battlegroup when they spawn. Those new units will catch up to the battlegroup and fight with them.
I've been hoping they would consider an ability to auto-request reinforcements, but that might be a bit overkill in the "let the computer do stuff for me" department.
