When creating or managing a practice, you can select which groups will be allowed to attend the practice. Only the members of those selected groups will be allowed to self-indicate whether or not they are attending the practice.
The groups allowed to attend practice can be changed from the Practices -> Manage page by clicking on the Groups Allowed button and adding / removing groups. On this page, you can control which groups can self-indicate whether or not they are attending practice as well as control how far in advance member of the group are allowed to set their attendance an how close to the start of practice they have to have their attendance plan set. If a athlete is a member of multiple groups allowed to attend the practice, they will inherit the attendance start time furthest from the start of practice and inherit the attendance end time closest to the start of practice based on the group settings.
If a coach needs to add or remove an athlete from a practice outside of the allowed attendance start/end times, they can do so from the Attendees tab on the lineup builder.