MIgration from Slack

I've just heard of Mattermost and I am very interesting to start using it in a team where people are actively using Slack already, to eventually convince people to migrate.

I'm not really sure I understand the compatibility feature beyond the Slack account import:

• Can I participate with a Slack team from Mattermost (I mean basically sending and receiving messages in a team channel)? Can Slack users participate in a Mattermost team?

If not, is there a way to do a gradual transition rather than an abrupt switch?

thanks