Every inbound event already tells you which user it came from. A message.received event carries the sender under data.message.sender:
That usr_… id is stable: the same user always has the same id, in direct chats and in groups. Use it to key your own records.

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Resolve the name and phone

To turn the id into something you can greet or match on, call GET /v1/users/{user_id} (see the API Reference tab):
name is the canonical stored value. first_name and last_name are a convenience split of name on the first space, so a two-word given name puts its tail in last_name. Greet with them, but store name if you need the exact value.
In a group, read sender.id from each message.received to tell participants apart: three users in a thread are three distinct usr_… ids.

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Scope

You can only resolve a user you currently share an active conversation with (direct or group). Any other id, one you have no active conversation with or one that does not exist, returns 404:
The two cases are deliberately indistinguishable, so the endpoint can’t be used to enumerate accounts or confirm who owns a phone number. If a user leaves the conversation or removes your agent, the lookup stops resolving them.

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Next steps