I have a pink grapefruit tree with bunches of small fruit on it. I would like to know if I should thin the fruit? Thanks.

Mother nature will thin the fruits as needed for the health of the tree. If this is a VERY small tree I would thin by hand in a few months if they haven't already fallen off. Citrus will start to form way more fruits than the tree can bring to maturity and then drop as many as needed to only keep what it can support. Hopefully this makes sense.