One way to express a short form of Sympathetic Joy is, "Isn't' that wonderful!" This is an expression of joy. As I said in the introductory talk, the Thai people say, "Anamotana," meaning "what good actions!." So that could be a phrase if you want to learn what it means. "Anamotana, anamotana!." Go around thinking "Anamotana" whenever we see something good happening in the world, people doing good things. The English version could be, "Isn't that wonderful!."
The other part of the question was, "I wasn't always sure when to say the Sympathetic Joy phrase, especially when one thought sparked off another about the good qualities of the Sangha."
Yes, sometimes that happens, especially when we start to reflect about the things that we see which are good. Joy starts to arise and we don't really want to put any words to it. So sometimes it's okay to do a free form Sympathetic Joy for the Sangha, without a Sympathetic Joy phrase, especially when you are just doing a contemplation of the Sangha. You don't have to actually bring up a phrase at that time, because you are going into just contemplation rather than just Sympathetic Joy at that time, but it may be good to just drop in a few "Anamotanas" occasionally. If you are actually trying to do Sympathetic Joy meditation where it has a form, then after we reflect about any particular people, then it is good to do a Sympathetic Joy phrase.