According to the answers, I realized that if it is possible, then by using MS Graph API.
As a last resort, I will try to find a necessary user among members of some team, something like this:
using (var connectorClient = new ConnectorClient(new Uri("https://smba.trafficmanager.net/emea/"), _microsoftAppCredentials))
{
var members = await connectorClient.Conversations.GetConversationMembersAsync("ID of team where installed bot and where may be a user");
...
Еhough this option is not entirely suitable for me, because:
- Bot should be installed into this team
- User may be not a member of this team
- User can leave this team
So I can't say, that my issue was resolved, but, I guess, this thread may be closed.
Thank you!