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You can see a list of everyone who is in one the groups you are in by clicking the My Contacts tile on The Hub homepage:
Or select MY CONTACTS from the Groups screen. That shows this screen:
You can only see people's email address and/or phone number if they have chosen to show that to members of a group you are in. You can click on a link to message or email someone, or click elsewhere on the person's entry to see more detail:
You can see this person hasn't used Mattermost for a while, but they have put in an External Contact link, so probably best to use that to contact them.