If I am in a group discussion, how can I give my opinion with out hurting others? Is it good to use the phrases "I think" or "personally speaking"?
There's no simple answer to this question. It depends on the personalities of the people in the conversation, how controversial the subject under discussion is, how forceful you want to be, the setting of the conversation, what others have recently said, and many other factors.
For example, if you are having a casual conversation with co-workers, everyone has been talking about the weather and vacation schedules and other bland topics, and you suddenly said, "Anyone who belongs to political party X is a Nazi and a moron", you would likely offend many people there. Even if none of them are members of X, they might have friends or relatives who are. Even if they don't, the statement is rude.
Adding an "I think" or similar words wouldn't really help to soften such a statement.
But an equally forceful statement about other subjects could be totally inoffensive. Like if you said, "I think strawberries taste awful", it's unlikely anyone would be offended or consider you rude. Maybe if someone there is a strawberry farmer he'd be annoyed, but in general that would be a totally "safe" statement.
Statements that would be extreme in a casual conversation among friends can be totally appropriate in a setting devoted to debating a particular subject. For example, if I was chatting with friends at work, I'd be unlikely to say, "Religion Y is based on irrational speculations." But if I was a speaker at a forum on comparative religion, such a statement could be appropriate.
Weasel words like "I think" or "It seems like" can help to soften a harsh statement, but in my humble opinion, not much. "I think you are ugly" is not really much softer than "You are ugly". Personally I bristle when I hear phrases like "some people say that", because most of the time this is the speaker's way of avoiding responsibility for his own statements. He attaches this phrase to what is in reality his own opinion, and then if too many people object he says, "hey wait, I didn't say that was my opinion, just that I've heard people say that." (You hear this on TV news all the time. The reporter will say, "Many people are saying that ..." followed by some controversial statement. Who are these "many people"? Usually the reporter and his friends. But by saying "many people say" he makes it sound -- or at least he supposes he makes it sound -- like he is reporting on a national trend rather than just expressing his own opinion.)
In my humble opinion, it is better to start with provable facts rather than broad generalizations. Like, "The news reports that Senator Jones has been indicted for accepting a bribe" long before you say "all members of party X are corrupt".
A lot depends on your goal in expressing an opinion. If you're just trying to have a friendly chat, you're better off to steer clear of controversial subjects. If you're trying to convert people to your religion or drum up support for your favorite candidate, then it's very hard to make your point without offending some, and you just have to accept that.