Hi,
do you mean something like this?
@startuml
a -> b
group first box
b -> c
group second box
c -> a
a --> c
group third box
c -> c
end group
c -> a
end group
c --> b
end group
a -> c
@enduml
Update: As you changed the topic and content of your question... multiple boxes to group participants are not (yet) possible.
BR,
Michael