Quentin There are no groups that you can put money into. There are Slice groups, however. These are not joint bank accounts, but they can be used to record transactions that have been made, in order to keep track of who paid what and how much. Members in a Slice group can then later settle their expenses, but nobody can just take money out of the accounts of others. Everyone has to accept it. Also keep in mind that you can add only bunq members to a Slice group. It's ok if they have free Travel memberships, but they have to sign up to a bunq account in order to be a member of such a group.
If you want to create real Joint bank accounts with other people at bunq, that's also a possibility but then everybody has to have at least a Premium membership, so that wouldn't work for your type of subscription. But the Slice groups are definitely a handy tool to settle expenses with friends and family, so give it a try if you can ;-)