soul cubes can be used to purchase pretty good weapons from the shop for 299 soul cubes (must be of the same soul cube, ex: 299 aqua.).
the weapons are located under the "premium gear" section of the shop, which is under "vip," and are located at the far right of the "premium gear" section.
you probably can no longer use your teammates because they got to a level that is too high for them to be used by you. as you can imagine, itd be quite unfair for someone who is, for example, level 5, to have a level 30 teammate to help them, whilst someone else only had their level 5 buddies to back them up.