Use the word "alumni" to refer to a group of people of both sexes (or where the sex of the group members is unknown) who have graduated from Memorial University. It is the most commonly used form and is the Memorial preference.
Alumnus refers to an individual male graduate.
Alumna refers to an individual female graduate.
Alumnae describes a group of graduates known to be comprised entirely of females. It is not commonly used. However, it should be used when the grads are known to be all female.
Alums is used as an informal substitute for alumni in spoken English, but should rarely be used in written form.