There are several steps an Administrator can follow to troubleshoot issues with households not being able to accept invitations:
Check the registration open/close dates under program/season setup; be sure that the season is open. An open or closed date may also be set for the grouping the player's team is attached to.
Check to be sure the invitation status for the player is set to "invited."
Search households to determine if there are duplicate households/members. If search results show there are two or more households, merge households into one in Settings > Data > Merge Data.
After you follow these steps, if you are still having trouble figuring out why the family can't accept their invitation, please contact the Support team.