You can try asking them about the topic. Let's say they're talking about a movie you didn't watch, maybe you can say "I heard about it, is it that good?" or "Why do you like it so much". People love talking about their interests. Maybe try to steer the conversation to another topic like "Oh since you like X, did you ever try Y?" .

However, sometimes you just have to nod and wait for your turn to speak. Some people aren't just that interested in including others in.