有Ｄ怪怪地，我阿姐叫我用pleasure

We have the pleasure to inform you that, to celebrate (somebody's) birthday, we will be hosting a birthday lunch at (place) at (time) today. Please inform us if you would be able to attend.

We have the pleasure to ...
We are pleased to ...
It is our pleasure to ...

... if you would attend
... if you would join us. (join is vt, and so you need an object.)

用pleasure會比please親切得多
但係我就會用join instead of attend.

但係極冗長
對唔住.我想re-write

we have pleasure to invite you to join XX birthday lunch at XX today, kindly reply your availability. cheers,

[ 純粹補充、不算作答、選答案不要選這個!]
樓上的 tintin852 已經給了最「地道」的修改, 僅分析如下:
[原文] We have pleasure to inform that in order to celebrate (somebody) birthday. We will have a birthday lunch at (place) at (time) today. If you will not attend. Pls inform by return.
[tintin852] We have the pleasure to inform you that, to celebrate (somebody's) birthday, we will be hosting a birthday lunch at (place) at (time) today. Please inform us if you would be able to attend.
1. 用 We are pleased 是另一種表達方法而已. We have pleasure 也有人這樣說, 但不及 We have the pleasure 正統.
2. 此處 inform 後應該如 tintin852 般加 object.
3. in order to... 「為了...」後面亦應該加要做甚麼, tintin852 給你把兩句恰當地連起來了 「為了慶祝... (所以)舉辦...」. " to celebrate" 之前加番 in order to 也可以啊~
4. we will have... (我們會有) 不及 we will be hosting... (我們會舉辦...) 表達的意思那麼清晰.
4. 在一般邀請函中很少會說 will not attend 「如不會出席」咁"崛", 多數會婉轉的說「如未能出席」即 are unable to attend 或如 tintin852 所寫的 would not be able to attend (相信他只是打漏了個 not 字). 另外, 是否不能出席才要回覆? 你可考慮一般做法, 即叫人"到唔到都回覆"; 若然如此, 則可簡單用 RSVP 來代替 "If you would not...".

2006-12-01 21:55:06 補充：
真係要拗呢就 grammar 冇錯. 但 will not attend (唔會o黎) 是中文講法, 英文會用 will not be able to attend (不能o黎) 婉轉些. ”by return” 是多餘.

用 pleasure 可解作高興

用 pleasure 雖可解作高興、快樂 , 但語態較恭敬 , 若邀請朋輩出席生日會 , 用 pleased 也可以了。

We are pleased to invite you to (somebody) 's birthday lunch tody at (place) at (time) .

If you will not attend. Pls inform by return. 這句在語氣上似乎不夠友善、客氣。可試試以下的寫法 :
Would you care to join our little party? Please let me know if you cannot come.

There are quite a lot of problems. Better use the following:

We are pleased to inform you that we will have a lunch to celebrate (somebody) birthday at (place) at (time) today. RSVP, if you could not join.

"have pleasure to" normally will be used give appreciation to the special guests

"will not attend" means not to attend by intention which is rather odd.

如果你用have 就會用pleasure ，因為pleased 係pp 而pleasure 係noun，所以在have 此verb 後應該係用noun instead of p.p.
如果你想用pleased, 你的句子應該係 We are pleased to inform .....