You’re planning to “experience” Europe in a hotel room? Most European hotel rooms are very small — you’ll sleep in your room, but probably won’t spend a lot of time there otherwise. Split into 2 and 3 for rooms, or stay in hostel dorms where you can often all be in the same room and may have others with you, too. I would not add another to a group that’s already pretty large for traveling just to have an even number UNLESS having an extra person means you’re more likely to split up for sightseeing as well as sleeping.

5 is a pretty large group for doing everything together. Plan to split up into singles at least part of the time — you’ll have a better time and are more likely to still be friends when you get back if you aren’t crammed together 24/7.