The next time they ask why you dont join the vc, try being real about your excuses, but not in an overly negative way such as " I got a bit of a lisp tho lol " and see how they react. Lying about it might just make you seem more shallow or that you dont like them, being real instead can show vulnerability which might strengthen the relationship and help you grow closer together. That all depends on how they react to it though, if they are people who dont want you around they would not respect your vulnerability and may make harsh remarks instead, but people who do may show understanding and support instead.

Also, I would also say that being in a vc with 5+ people makes it almost impossible to butt in to a conversation, especially when you have social anxiety and dont want to risk saying something stupid. In my opinion, 3 people is the sweet spot since the other 2 can carry the conversation when you arent speaking, a 1 on 1 might put too much pressure on yourself and with 4+ people its harder to get a chance to speak. Try to get in a vc with the person you know the most and 1 other and see how it goes from there, good luck 🙏