When a user is member of two groups with overlapping rights, read only right overwrites read-write right.

Explanation:
usergroup1 has read-only access for every hostgroup.
usergroup2 has read-write access for hostgroup "XYZ"

User A is member of both usergroups but he has only read-only access for hostgroup "XYZ".

Why doesn't he have read-write access for hostgroup "XYZ"? That's what I would expect if I can assign multiple usergroups to one user.

Here is also a forum thread where someone else specified it:
http://www.zabbix.com/forum/showthread.php?t=16755