In order to RSVP on behalf of someone else, ther are two basic requirements:
The other account needs to be linked to your account
You must be on the roster of the team that the linked account is on. For example, if you need to RSVP for the events on the schedule for your child's team, you will need to be added to the roster of that team. Typically, this is done by adding you as a "Parent", although that is ultimately up to the coach/team administrator. Generally speaking, you just need to be active on the team roster in any capacity so that you can access the team and RSVP for any linked accounts.