I'm not fond of the "demon as party member" concept, but I think summoning would be a nice way to interact with Tartarus.
I would suggest the following limitations:
- the demon cannot go far from the invocation pentacle.
- drawing a pentacle takes time (no instant summon).
- invocation pentacles have a maximum duration.
- an optional containment circle can be drawn around the invocation pentacle to prevent demon from moving and PvP. This circle prevents item exchanges too.
- rare demonic consumables or items have to be used (for example, owning one of demon body parts. PvP should be the way to obtain those).
- any item exchanged through summoning is bound to the invocation pentacle duration.
I think summoning would be interesting simply to chat and arrange game events (an evil human could designate a specific target to be destroyed during an eclipse? arrange interplanetary trade?). Also, there are many form of help a demon can provide to a summoner (some Tartarus specific buff spells, for example?)