A non skyblock rank player is able to join one profile with his friends, this is referred to as a 'co-op' profile and is the second slot. The perk of having skyblock rank is that you're able to have 3 co-op profiles instead of the usual 1.
All of this means that you can invite your friends IF they do not already have their one co-op filled. If they do they'd need to leave that first.
Lyriie typing fast yeez