"To join the group" means to become a part of a specific group or team by actively participating and being included in their activities and conversations. Full definition
Each month, I have the privilege of joining a group of talented and creative bloggers to share the joys and experiences of that particular time of year and what it means to us. (dukemanorfarm.com)
You're immediately joining a group in which you all share a love of something. (datingadvice.com)
If you're at a complete loss, try joining groups for alumni of the veterinary schools your staff attended. (lifelearn.com)