Please note my corrections above. Your member profile shows that you are an English teacher. As such, please try to ensure that you write correct English at all times on the forum.
"Will you join us for tea?" could be viewed as both an offer and an invitation. You are offering the listener the opportunity to join you for tea. You are also inviting them to join you for tea.
Note that I have changed your thread title. Titles should include some/all of the words/phrases you are asking us about.