You can get ID90s(90% off) for yourself and a partner, you have to prove you live together. If you do live together then they can travel independantly from you, if you don't then they must be put down as a travel companion and they have to travel with you. If you don't have a partner then you can nominate one friend as a travel companion. Or you can put down 3 family members, mum, dad and sister etc...... You get these after 6 months service.
ID90s are standby tickets so aren't great, especially over school holidays as it goes on seniority. I have been with BA 4 years and only used staff travel twice as it can be a real pain in the bum. If you don't mind where you go and what time you leave you maybe lucky and get on a flight, but if you need to be back at a certain time then you usually have to buy 'back up' tickets on other airlines to make sure you get home.
You can get discounted fares for friends and family, these are called Hotline tickets, they are discounted tickets, but usually only have a few quid off. Usually if you shop about on the net you can find them cheaper anyway.