Hostess with the Mostest
Like the wedding itself, the responsibility of financing and planning an engagement party traditionally falls on the parents of the bride.
If for some reason the bride's parents can't host the party (or simply choose not to), the groom's parents or any other family member or friend can step in.
So, what if no one is offering? Just make it known that you'd like to have an engagement party, and somebody will most likely step up. Of course, there's also nothing wrong with throwing your own celebratory shindig.
If you have multiple friends and family members proposing to host the bash, put them in contact with one another and ask them to co-host it. Remember, these people will be seeing each other again (and again) at wedding-related events, so you want make sure everyone remains amicable. Plus, the more the merrier -- at least in terms of the party's budget!