From:
https://www.1914-1918.net/msa1916.html
Men were allocated into a Class, which was connected with the year of their birth, and were notified that they would be called up by Class.
Class 1 was for those born in 1897. They were 18 years old. They were told they would not be called up until they were aged 19. Class 2 was for those born in 1896, Class 3 for 1895 and so on up to Class 23 for those born in 1875.
A Public Proclamation was placed in prominent spots, advising the public the date on which a particular Class would begin call up. This was deemed to be sufficient notice, but in additional generally each man received an individual notice. It was the individual's responsibility to be alert to such notices and to report himself for duty. There were penalties for not reporting and for inducing or assisting a reservist to absent himself.
The Groups for single men closed at midnight 1/2 March 1916 and from that time all legally eligible men not exempted were deemed to have passed into the army reserve. It was not necessary to attest these men but their basic details had to be entered on the enrolment form B.2513.
ACI 379 reminded Area commanders to provide medical facilities for the examinations necessary to complete enrolment. Men who failed the medical and enrolment were told they would not be needed again; those who passed were told they would have another medical when called up.
Men of Class 1 (that is, 18 year olds), once enrolled, were given the option of returning home or remaining with the Colours and undergoing special training until they were 19. (ACI 839, 18 April 1916)
Call up under the Derby Scheme began: Groups 2 to 5 were called up in the last two weeks of January 1916, and Groups 6 to 13 in February. The last single groups other than the 18 year-olds were called up in March. This last batch were called up in parallel to the first men to be summoned under
conscription. The recruits were not necessarily posted to their local regiments and from this time one it is not wise to assume that a man would go into his local regiment.