According to Rabbi Moses Isserles whose Ashkenazi glosses adorn the Shulchan Aruch, it is “the custom in these countries” for many people to be called to the Torah on Simchat Torah (Orach Chayyim 669). Some synagogues call up every male worshipper. Other congregations call people up in groups – all the Kohanim, all the Levi’im, and then the Yisra’elim in groups.
The principle behind the multiplicity of call-ups is that the Torah is morashah kehillat Ya’akov – “the inheritance of the (whole) Congregation of Jacob”. One of the group Aliyot is reserved for Kol HaNe’arim – “all the children”, a phrase from II Sam. 13:22. Generally the call-ups are in the morning, but some congregations read the first portions of V’zot HaB’rachah on Simchat Torah evening, using the Rosh HaShanah and Yom Kippur melody. There is a custom in some places of giving these evening call-ups to boys who have become Bar-Mitzvah during the year.