See also the official documentation:

• For group owners and moderators: Help Center > Group Owners Manual > Taking donations and reviewing payment activity
• For group members: Help Center > Members Manual > Responding to donation requests in Premium and Enterprise groups

Premium and Enterprise groups can request donations from their members.

There's a Donations tab under the Admin menu, where you can create and manage donation requests. Groups that have open donation requests will see a Donations tab in the normal group sidebar where members can view open donation requests.

Setting up Stripe with Admin Donations page on Groups.io for the first time:

Groups.io uses Stripe.com to accept credit card payments.

In order to accept donations, you will need to have a Stripe account. Stripe accounts are free.
Clicking the Connect with Stripe button will take you to a page to create a new Stripe account or connect an existing Stripe account. 

Once you've connected a Stripe account, you will be able to create donation requests and your members will be able to donate using a credit card.
When you create a donation request, it is considered open, which means that members can submit donations. You can close donations when they are complete.

Go to Admin, Activity. Payment Activity button. Dropdown menu.

The Groups.io processing fee is 5%, which includes all credit card processing fees.

Note: Donors will be donating to the group where they use the Donations page/link.  "I suspect all of the donations would go into 'one pot' unless you create a Stripe account for each group.  Hopefully, the Stripe account would specify which group it came from." (2019-11-10)

Setting up a Stripe account:

Open an account with Stripe and giving it the Account Name of [MyGroupName], the Legal Business Name of [MyGroupName], along with the Statement Descriptor of [MyGroupName Donations] and associated my website as: https://groups.io/g/[MyGroupName], as well as other personal info,

Attach/add a bank account and set up a Direct Debit:

• Add a Bank Account and set a Payment Schedule under Settings > Payments and Payouts > Bank accounts and scheduling.
• Adding the bank account requires you to set up a Direct Debit arrangement (to receive payments) and then a Payout schedule.
• You can select an Automatic Daily, Weekly or Monthly schedule, or a Manual schedule.
• The money goes to your connected bank account. 

Donors -

Do members need a Stripe account to donate?
No.

Can the credit card data be entered from a keyboard or keypad?
Yes, just type in the numbers and other information like you would on any site.