There are two ways to receive squad messages.
1) An administrator assigns you to a squad with a squad-level role such as coach.
2) Your user account gets linked to the participant profile of someone on the squad's roster.
This happens automatically when you register a participant. If someone else registered the participant, they need to add you to the profile. If they add you as a parent of the participant, by default the system will also link your user account to the participant. However, the person performing the registration does have the ability to turn this off.
At any time, administrators and users already linked to the participant profile can add you. For more information see Who can manage a participant's information and receive emails relating to them.
Here is a video detailing how to manage Linked Users.