You can invite as many team members (both internal and external) to a group as you like.
If you are less than 10 people in the group, you'll see the profile images and online status of all people directly in the top bar.
If you are +10 people in a group you can press the "+10" icon to see who's part of the group and who's online.
Note: If you are having a large group, consider if you should split it up into smaller and more agile teams. Maybe you want to keep the overall group for more general communication to between your team members.