If child is nervous around new people, try to meet them in places where there is lots of space to move away and still have fun. EG a park rather than going to someone's house for tea. That enables a child to play on the swings/slide/on the grass verge 50 yards away and begin interacting AT THEIR OWN PACE (Which might mean - not actually directly till the next play date)
Some children might like to be told ahead of time who they are going to meet - perhaps see a photo too.
Some adults are REALLY bad about invading the personal space of a child who is not comfortable with it. In that situation, defend the child - make a physical barrier between yourself and the adult so that they would have to invade your space to get at the child. I know parents who say explicitly "give him/her some space - (s)he takes a while to be comfortable with new people".
I also know parents who say "please just ignore my small child until (s)he gets used to your presence". Some children like to be around people without 5 pairs of eyes looking expectantly at them waiting for some kind of performance.
The other thing is that I think parents need to be willing to follow their child's cues about pace of almost any activity. It might take 20 visits just playing on the side of the swimming pool before another 5 where the child plays in their depth and then eventually wants to explore further - but sides of pools are wonderful places to play and so are the steps in and out. Rather than thinking "my child is timid" think "my child wants to get every ounce of learning and richness out of every aspect of this before the next bunch of sensory overload bangs in".