Followers of Majora are the collective name referring to all units from the factions which have converted to Majora Worship.
In addition to its unholy spawn, Majora's army is composed of many followers from a large variety of races who have willingly put their faith in the dark god. Any unit of any faction can potentially appear in the ranks of Majora's army, and with them, all their strengths are brought to the field of battle.
The Church of Majora can recruit all units from all factions which have been converted to Majora Worship (it is unkown if the units from the Kingdom of Ikana or the Forces of Twilight, which both start already worshipping Majora, can be recruited or not).
The follower units work exactly like the respective units of their empires. For example if Majora recruits Hylian Crossbowmen they function just like Hylian Crossbowmen, or if they recruit Goron Guardians they function just like Goron Guardians. The only difference is the texture.