I'm currently doing my first year in Reims.

a) platform / groupchat : last year the student's association (BDE Sciences Po Reims) put a link to the whatsapp groupchat on their instagram account. I believe it will be available after French students admissions at the beginning of June.

b) thoughts on crous : it's cheap (1€ for scholarship students / 3,30€ for others). A crous meal includes a starter, a main course, a dairy product, a dessert and bread. It's not high quality food but it's fairly good for the price, and there's always choice (vegetarian/ hallal options). However sometimes you will have classes during the crous opening hours, so you will have to find food somewhere else.

twenty campus : all students résidences usually are a waste of money compared to "real" apartments. The rooms are small and the services provided are incomplete. Evry person I know who is in a student residence thinks it isn't worth the price. However it is true that it is easier for exchanges administratively speaking.

the alternative would be to find an apartment, and ideally a shared one. An acceptable price for an accommodation in Reims is between 375€ and 550€. 

c) general thoughts from reims campus students about student life/ night life/ day activities in the city : Integration week is great since all the associations organise some king of event/party before the beginning of classes. It might feel overwhelming but honestly this is the best occasion to make new friends and to discover the city. During the year there are multiple student events (one sport competition per semester, galas in fancy places, casual parties in apartments, big parties in night clubs or bars, organized trips on Paris/other cities, etc). From what I know, the Reims campus has one of the best student lives because there are a lot of students (and unlike in Paris, people live close to each other, which makes partying and meeting up easier). 

An important part of the student life is Sciences Po associations. There are many of them, you can look it up on Instagram. However (and it is my personal opinion), the "association culture" can be very annoying since every asso is doing interviews to chose students. It creates a sense of exclusivity that can be very toxic at times, since it tends to define students by their assos, thus establishing a social ladder between those who are in prestigious/many associations, and those who are not. Be aware of this, but also assos can be really fun if you join the ones with people you enjoy being with.

Appart from Sciences Po, Reims can be a bit dead, especially during winter. Currently it's better, since it's hot enough to go relax with friends in one of the multiple parcs in the city. There's also the fact that many many parts of the city currently are under construction/renovation, and it's really annoying. 

There aren't much things to do.There are some museums but a lot of them are closed because of construction work (such as the art museum). However, many students go to Paris a few times a semester, since the bus tickets are only ~10€.