Some suggestions of people to follow; in no particular order
Peter Liljedahl on Twitter
Maegan Giroux on Twitter
Jennifer Bay Williams on Twitter
Tim Brzezinski on Twitter
Christina Tondevold on Twitter (Build Math Minds)
Graham Fletchy on Twitter
Robert Kapalinsky on Twitter
Pam Harris on Twitter
Mary Bourassa on Twitter
(9-12)
Desmos on Twitter
Howie Hua on Twitter
Mathigon on Twitter
Just a smattering of groups that offer ideas and tasks
Building Thinking Classrooms: K - 2
Building Thinking Classrooms, a grade 3-5 collaboration
MMMathMania
Lots of game ideas!