翻译：很久以前就开始关注贵司了，贵司同事也曾经我我司展会参观过。

Long ago i began to pay attention your company, and your company has also been my colleagues in the Division I exhibition visited.

I have been paying close attentions to your company for a long time, and your collegues have visited our exhibition as well.

I paid close attention to your company long time ago. And your colleague have visited our exhibition.

We've been paying attention to you for a long time. Your colleague has also come to visit our booth before.
用have been paying，表示已经并正在关注~~