私も参加するよ。ただしみんなが参加するって言ったらねって英語でなんて言うの？

To say you will join in on an event you can also say i'll participate/join or take part they all mean the same.
By saying 'as well; it means others are too (as well as me)
So you can say I'll join in/take part/participate/take part) as long as everyone else does as well'

I say you will do something only if everyone else or others go/do as well, you can say "If everyone else goes, I'll come too."

The two sentences you see provided above are terrific ways to express to your listener that you will participate in something as long as everyone else participates as well. In the first sentence you will see the phrase follow suit. This phrase means to follow another persons actions. This is a phrase that is common in our everyday conversation and it would make a great addition to your vocabulary.

The above format is the most normal in the UK. It is a standard phrase and uses the First Conditional structure to indicate an option dependant upon a condition.
Form: If + verb in present simple + Will (or future tense)+ verb

All three of these are acceptable ways to express the same idea. A way of saying that you will participate is to use the phrase, "join in," which indicates that others are already engaged in an activity and you will now engage in the same activity with them. Also, saying "so will I" shows that you will do the same as someone else or as a group.

I will go with the flow.

Im in if others are

Count me in with the group

When you want to explain that you would only go to an event/party if others are willing to go too; then you may explain it in the following ways:
-I'll go if everyone else does too.
-I will consider going if everyone else is going too.
-I won't go if everyone else doesn't. I"ll only go if they are too.

These are two pretty simple options to express what you want to say. I personally use the first option, although, there is no big difference between them, so, choose any.