How do people join?
Send them one invite link. They create an account or sign in, then you choose the projects they should see. New members start with none. If the link wanders outside the team, make a new one. The old one stops working.
Not everyone needs every project
Invite the people who help fix errors, then choose which projects each person can see. Admins can look after the whole installation. Members see the projects they actually work on.
Who should be an admin?
Keep admin access for people who need to manage the installation or see every project. Everyone else can be a member with access to the projects they work on. You can always promote them later. If you invite clients, give them member access to their own projects. Admins can still see every project; project permissions are not separate servers.
Can members still do the actual work?
Yes. In the projects they can access, members can inspect and resolve errors. The installation-wide controls stay with admins.
What happens when someone leaves?
Remove them from the team and their access stops. Telebugs signs them out and turns off their project notifications. If they come back, invite them again.