我认为，填joining 或joining in 都正确，只是意思不同。

前者表示“参加远征探险队伍”；后者表示“参加远征探险活动”，因为expedition 有此两种含义。

远征队；探险队：

You can refer to a group of people who are going on an expedition as an expedition.

远征旅程；探险活动：

An expedition is an organized journey that is made for a particular purpose such as exploration.

选A：他对参加远征探险队仍在犹豫不决。

选B：他对参加远征探险活动仍在犹豫不决。

He’s still hesitating about ______ the expedition.

A. joining   B. joining in

请问如何选择？join the expedition还是join in the expedition？

He's still hesitating about joining the expedition.

参考译文：他对参加这个探险队仍犹豫不决。

expedition  n.远征队;探险队(英语解释： You can refer to a group of people who are going on an expedition as an expedition .)

He is the commander of the expedition.
他是探险队的队长。

Forty-three members of the expedition were killed.  

探险队中43名成员遇难。

回答如下：

应该选A为正确，原因如下

(1)join意为“参加/加入某个组织/机构/团体/俱乐部，或者参加到某个人群中去，从而成为其中一员”

She joined a swimming club.

她参加了一个游泳俱乐部。    

We both joined the Labor Party. 

我们俩都加入了工党。  

He also asked to join the army.
他也要求去参军。

Nothing would tempt me to join the army.
什麽也不能引诱我参军。

He lied about his age to join the army.
他为了参军谎报了年龄。

(2)join in 意为“参加”后面接的宾语一般是表示竞赛、娱乐、游戏等活动的名词或动名词形式。 

Can I join in the game?

我可以参加这个游戏吗？

They all join in singing the song. 

他们一起唱这首歌。

Will you all join in singing the refrain, please?
请大家一道来唱副歌好吗?

They pestered her to join in the scheme.
他们不断要求她参与那计划。

Can you see the join in the coat?
你能看到衣服上的接缝吗？

I will join in the project, heart and hand.
我会满腔热情地参加这项工程。

Can I join in (the game)?
我参加(这个游戏)行吗?

We want to join in the masquerade.
我们想去参加化装舞会。