Check that they have joined the team
Go to Settings > Teams then select the team they have been invited to. Look under their email and see if it says Invited or Joined.
If it says Joined, they have access to the Social Sets.
If it says Invited, they do not have access and it's most likely because they have not created a Pallyy account yet, or they created a Pallyy account with a different email to the one you invited.
If they created a Pallyy account with a different email, tell them to log out and create a new one with the correct email.
Check the Social Set menu
Sometimes they may have access, but can't see where to access the Social Sets in the team.
Ask them to open the Social Set menu which is located in the top left of the dashboard.
They should see the team name and under it will list and Social Sets in that team. Click a Social Set to switch into it.
Check you have added the Social Set to the team
Go to Settings > Teams then select the team in question. Look for the Social Set under the Connected Social Sets section.
If it's not there, you'll need to connect it to your team so your team members have access to it.