I notice that there are you can query object with tags.
But how do I get tags associated with the objects in the first place?
By adding tag to Content File you say that users with this tag will be able to retrieve this file with query
[QBContent taggedBlobsWithPagedRequest:pagedRequest delegate:self];
To update user's tags use
QBUUser *user = [QBUUser user];
user.tags = [NSArray arrayWithObjects:@"man", @"travel", nil];
[QBUsers updateUser:user delegate:self];