Cell group allocation is based on your district. So if you stay
in the east, you belong to a cell group in the east. This makes it
easier to meet for cell group meeting and worship, which usually
takes place in a member's house.
A cell group is the smallest unit in a big church (unless they have
buddy system? lol). During cell group meetings, they conduct bible
study sessions and worship. Worship as in singing songs accompanied
by the guitar. On top of that, they could be having birthday
celebration for the members or personal admin, like submitting
forms or promoting church events.
Note that a church member can hold multiple roles in a big church.
You belong to a cell group and you are encouraged to join a
ministry. You could be an usher, a performer, an in-charge for
small kids etc. for the latter.
A cell group is practical for big churches also, in the sense there
is coordination to chop [good] seats during the service.