It's an Apple thing, and there's not a lot you can do about it (unless you switch to an iPhone). Apple's iMessage is a closed, proprietary messaging network. While iMessage users can receive the industry common SMS and MMS messages, in order to send them they have to intentionally opt to do so, otherwise the iMessage app will always default to the using iMessage format messages. That's why you're only partially engaged with those group texts, it's a matter where everyone involved would have to specifically include you into the mix with every message.
Apple won't allow or release an Android version of iMessage, it prefers to keep its messaging system only within Apple's ecosystem. There are some third-party, non-Google Play apps that will let Android users become part of Apple's iMessage network but do note these are purely independent ventures, and you'll need to have and give your Apple I.D. and password to an anonymous site as Apple services require a valid login.