It is not possible to use a promptset to define an Agent Group because the Agent Group is resolved before the prompt is done.
You can work around this using the script below on the pre process tab:
:SET &IS_OVERWRITE_GROUP# = GET_OBJECT_TYPE("&OVERWRITE_AGENT#")
:IF &IS_OVERWRITE_GROUP# = "HOSTG"
: SET &HND# = PREP_PROCESS_HOSTGROUP(&OVERWRITE_AGENT#,"*",BY_RULE)
: PROCESS &HND#
: SET &OVERWRITE_AGENT# = GET_PROCESS_LINE(&HND#,1)
: SET &STATUS# = GET_PROCESS_LINE(&HND#,2)
: PRINT "HEADER: overwrite agent with agent from AGENTGROUP: >&OVERWRITE_AGENT#< - Status: >&STATUS#<"
: IF &STATUS# = "Y"
: TERM_PROCESS
: ENDIF
: ENDPROCESS
: CLOSE_PROCESS &HND#
:ENDIF