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
Note: this is especially helpful when collecting RSVPs from ONLY players.