Conformist, as already answered is a good fit. If you are looking for a pejorative synonym, sheep is an alternative.
Macmillan:
sheepNOUN [COUNTABLE]
2INFORMAL someone who does the same as everyone else without thinking about it
ODO:
sheepNOUN
2 Used with reference to people who are too easily influenced or led.
‘Don't be a sheep and follow the flock, do something for yourselves, for God's sake.’