To bring guest to your show, I would recommend you TVU Partyline. That'd be my first choice. You have no limits in number of users, or I think they max it at 1000, the quality is great, and it's super stable using my house internet connection. It's entirely cloud based, and you can also mix the difference live guests on their cloud production platform TVU Producer. Your guest can join using a simple link, like Quicklink, from their phone, tablet or laptop, it works pretty well. Here you have more info https://www.tvunetworks.com/products/tvu-partyline/
They also have ways to make your output SDI using their physical receivers, I've seen they've done that recently for UEFA Champions League draws.