My little brother was exactly like that when he was younger! They just gotta get used to meeting new people. It took my sister's husband months for him to get used to being around without freaking out lol. Quite funny la, but kids are kids. We tried coaxing him into making friends coz "the more friends you have the happier you will be" but then we sorta went back on our words coz stranger danger hahahaha.
It's just stranger anxiety. You can tell the opposite party that your toddler needs time to adapt and accept them. Have a little patience in trying to get the toddler to accept them