You can create a group message from iMessage using any phone number and doesn’t have to be an iOS user with iMessage activated.
There is a nice guide from Apple Support about Send a group message on your iPhone, iPad, or iPod touch, With Messages, you can send a message to a group of people. You can also name your group, add or remove people from the group, mute notifications, or leave the group.
But there are no any information about removing from the group if the user is not an iOS user. This is appeared to be impossible.
You can only add and remove user from a group conversation on iMessage, can’t do it if there is a non iOS user in the group.