I don't think a larger lounge is necessary, just a little bit more organisation and information. So that any passenger arriving at P1 knows what's going on. Some relatively simple signage could explain that "club" passengers can go to the lounge, and that the highlander & lowlander are separate departures. It could also display the time at which general boarding will open, and ideally this would match the info on people's tickets. The crowd could be separated into different groups for each portion of the train even if it's not possible to start checking people in at that point.
In the past they've had various systems; I'm sure at one point there were little check-in posts with signs above. Now it seems to have degraded into an arrangement where most of the passengers don't know what's going on and some get anxious about whether they are even waiting for the right train.
CS passengers, especially berth, but all really, deserve a dedicated lounge area. Hanging around the bin movement area behind the buffers on P1 is a appalling way to treat people who've paid serious money for the trip. In inclement weather, when cold, windy or rain, waiting by those gates isn't fun.
A boarding time ( and alighting time) is displayed when booking, but it's not unusual for the CS set to leave Wembley late and be delayed into Euston. The hosts board at EUS and don't start their checks, key prep etc until then. Actual boarding time can often not be the advertised time.
What would be a good idea,is maybe having separate digital screens at entrance and on platform (controlled by CS) to show which platform zone their respective carriage is located. ( can show where: club, classic, seats, lounge, cycle, luggage etc.
What doesn't help is I believe there used to be more hosts, almost 1 per carriage where now they seem more thinly spread and while having CS staff at platform entrance is a good idea to offer help to those who need it, why they insist on checking every ticket to signpost where to go (only for ticket to be checked before boarding at relevant carriage destination) slows things down. especially for regular and semi regular passengers who know where to go.