"Welcome to my office" is probably the phrase you're looking for. You would say it when someone enters. "You are very welcome to my office" could mean the same thing but implies that the person is welcome to use your office any time. It would be inappropriate to use the past tense (welcomed) in these cases. You might later say, "I welcomed John to my office today and he didn't respond." Or something like that. Hope it helps.

The impression "Welcome to somewhere" implies that you stay at that place. E.g. "Welcomt to my hometown" means that you stay in your hometown and invite others to come to your hometown. But, when you're out in other place, for my part, I prefer to use "Somewhere welcomes you". Hopes it helps.

You are very welcome here. Welcome to my office. Make yourself at home. Come by anytime. Stop by and stay awhile. I welcome you to my office. You will be welcomed at my office. We're putting out the welcome mat for you.