The problem with the Guitar Star skill journal challenge is that it's partially bugged and it's poorly written. What it really means is that your Sim needs to complete 10 opportunities that are related to guitar skill. It's nothing to do with playing at a neighbor's party or anything. You just have to wait until you get 10 opportunities in that particular skill. It's also helpful if your Sim doesn't know any other skill because the skill slot in your journal will have some competition. If you have nothing but guitar skill, you have a better chance of getting only guitar opportunities.
It's definitely a hard challenge, but it can be done.