Currently in order to transport some group by transport - totally:
1. We need to select transport and move it to expected loading zone
2. Order each unit to move into transport
3. Wait for loading, then manually move transport to new position
This might become somewhat tricky when there are relatively low number of units (compared to opponents), which need to be frequently commanded to different locations - both for support and backup.
1. Allow loading to be chainable for transport: When some units are ordered to move into transport, this transport should become busy. It still may be moved without waiting units, but if this movement is chained, transport should wait while all units are loaded, then do next orders (it may be offloading order). Transport may force to become unbusy if there are no more place for lightest of ordered units - as option, units may continue movement to prepare for another transport.
2. If transport moves, units still may be ordered to move to it - they just need to move to transport's destination, wait (if transport is still approaching) and load. Following behavior of transport - by previous proposal. Similar behavior is noticed in second campaign, in civilian save mission, when civilians waited for collective's transport.
3. New unit control - load into specific transport. Just like for commanders, there may be row of transport related buttons.
Current default loading button may order to nearest available transport.
I'm unsure yet, what is behavior in this situation: either they move transport, while transport may wait or be moved to more convenient LZ with behavior, described above, or.....
4. Transport may have own control - auto-fly to ordered group. Arrival point is autoassigned to be center of all assigned units. Though after that transport still may be reordered to new LZ - with behavior from 2nd note.
++ And one unrelated proposal - since transport's defence is tiny, it needs some escort - would be nice to add VTOL function to defend it, besides of loading - in same way, as for ground units.