Question

My favorite group is Coldplay. Or my favorite group are coldplay.

Answer 1

Group is a noun, singular.

Groups is a noun, plural.

When speaking about a group of musical artists or performers, the word band is used.

e.g.: My favourite band is "Coldplay".

Answer 2

Group is singular as in "The group of boys was late for the party".

Groups is plural as in "Several groups of boys were late for the party"

Answer 3

My favorite group or band IS 'Coldplay' ...

BUT ...

word GROUP can be used both as singular and plural ...- this is very British and I would not expect to see it in American English ...

 Group as a collective noun can be followed by a singular or plural verb. It takes asingular verb when the persons or things that make up the group are considered collectively:The dance group is ready for rehearsal. Group takes a plural verb when the persons or thingsthat constitute it are considered individually: The group were divided in their sympathies.