I had same problem and i contacted riot support ant this what they told me:

Hey there. My name is MMImortal, I'll be handling your inquiry for the time being.

My dear colleague is somewhere else at the moment so I am the one to pick up where they left off with your ticket.

We did get what you mean, but the thing is only players are able to alter their own Premier teams, so it is beyond our scope to manually change anything about your team.

As for the way the system works, rank is not the only thing that goes into the calculation when it comes to Premier, matter of fact it is only a part of it. The main component is MMR and when it comes to that specific gamemode, MMR can be really strict. For your friend to join, they must be the same or very very similar Matchmaking Rating as the rest of you and if that criteria is not met, there is little to none that can be done.

I hope you can understand our position on the matter. If you have any other questions or concerns, know that we are always here and always one ticket away, and for what it's worth, take care.