same as with templates, group management in mass update should be expanded to have 'replace group membership' and 'assign additional groups'
Let's do a quick fix by renaming 'Group' to 'Replace groups'.
Fixed in development branch. Ready to test.
Fixed in revision 22840.
reopening to :
(1) set correct "fix for" versions;
(2) change strings further to obey 16 at https://zabbix.org/wiki/Docs/specs/syntax#Entities
"Replace groups" -> "Replace host groups" (although "Replace host group membership" might be a bit more accurate 
)
"New group" -> "New host group"
for trunk, fix "Relpace"
fixed in :
svn://svn.zabbix.com/branches/dev/ZBX-2504
svn://svn.zabbix.com/branches/dev/ZBX-2504-trunk