I'd just like to emphasize the idea of hooks. If you're going to include someone in your RP, then please, include them in your RP! It's not fair to invite someone in your conversation and then start talking about things they have no knowledge about or have previously had no involvement in. Take the time to explain the situation to them and catch them up if you truly want them to get involved. Otherwise, they have no real way of participating, and they sit there feeling like a third (fourth, fifth) wheel. Many people will happily get involved in your RP/plots if only you extend to them a simple hook. 
Give them something their characters might care about or be interested in.
Give them something their characters might care about or be interested in.
