Look at zoom.
The pro level allows multiple hosts, screen sharing, and up to 300 participants. It's not cheap -- $200/month. There is a free version but that would limit you to 40 minutes for a group meeting.
There's a level of Zoom that allows for up to 100 participants/1 host for only $15/month. zoom.us
In pre-COVID19 times, I would go to orchid societies, garden clubs, master gardeners & libraries to give presentations on orchids. Currently all such group meetings are cancelled, and probably will remain so for the next 12 months or longer.
I would like to try to offer these groups webinar presentations instead, but need some guidance: