I'm not a regular user myself but I know a few who are, so having picked up on the etiquette I think this is the best answer to your question second hand.
Very busy lounges, typically the northbound Inverness on Thursday and Friday nights in the summer, will fill at boarding time (2030). I'm not so familiar with people who travel in the opposite direction from Inverness, but I imagine the same thing will happen there and *probably* mostly on Sunday and Monday nights. I understand that in those circumstances some crew may ask lounge users for their compartment numbers on the basis that they're giving priority to First Class users, and Standard ticket holders may be asked to return later. Others are happy to work on a first come first served basis. Bear in mind that if it's that busy they aren't able to fit all First Class holders either and there may be tables reserved for people with prepaid meals.
The best advice I've been given is to be on your mettle, especially northbound late in the week and southbound at the start, get checked in on the dot and ready to go to the lounge straight away. Unsurprisingly being pleasant to whoever's in charge of the lounge does you no harm, so if you're travelling Standard and they're full, offer to come back later or help with a solution like temporarily spreading your group out rather than arguing about it and they may be able to work wonders for you like moving people from other tables.
Also don't be afraid to ask politely to share a table with another passenger if someone's on their own, the regulars are generally friendly, full of stories, and happy to chat, or some may even offer to move to accommodate a larger group. It's rude to ask them to move however and I'm told the protocol is that you owe them a drink if they do shift for for you. (I'm not sure how true this last bit is, the person who told me likes a whisky!)