In MS Teams, with each new interaction with the bot, a "conversation Update activity" is first sent to the bot, this is what triggers the welcome greeting. You get back the following context in the returned JSON - in the Turn.Activity memory scope.
...snip...
"serviceUrl":"https://smba.trafficmanager.net/emea/",
"channelId":"msteams",
"from":{
"id":"<<Unique MS Teams Identifier for the end user>>",
"name":"<<AAD firstname surname is returned here>>",
"aadObjectId":"<<GUID is returned here>>",
"role":"user"
},
...snip...
You can Use the aadObjectId to query the MS Graph using an HTTPRequest if you want to get further info. Or just use the persons "name" field (which is from AAD) if that is good enough.