When an activity doesn't require all members of your team, here's how to select certain individuals for the event:
Make sure you are the admin
Start in the Schedule tab at the bottom of your screen
Choose the "+" to create a new activity
Once you've entered the name, location and time of event, click Invitations
Choose the Select option, which will lead you to the team list
If you want to change an invite list for an existing event, just press edit on the top right corner of that event, and follow the same steps as above.
Anyone who is not invited, will see a "not selected to attend" notice under the event. They will still see the event in their calendar.
Note that by default, players, coaches and admins are automatically invited to all events.