Two-way text forwarding system

I represent a small organization. We are looking to have a phone number that we can distribute to any of our volunteers. The volunteers could send SMS or MMS messages to the number. Those messages would then be forwarded along to a small team. This is the first forwarding direction: volunteer to team through the single number.

Then, anyone on that small team could reply back to the sender. The reply (SMS/MMS) would appear to be coming from the single number that the volunteer had texted originally. This is the second forwarding direction: team to volunteer through the single number.

I'm not looking for a way to roll my own solution. I'd be happy to work with a company that offers a service with two-way forwarding.