There's no super easy way to merge two Meetup Groups, unfortunately. If you want to combine two groups, this is what we recommend:
Select one Meetup Group to be the main Meetup Group (1) and one Meetup Group to delete (2).
Go into Meetup Group 2 and set it so it will not accept new members, change the name to something like,"CLOSING DOWN THIS MEETUP GROUP. PLEASE JOIN MEETUP GROUP 1," and post a link to Meetup Group 1 in the group description.
Next, send a message to your members informing them of your intentions (i.e. shutting down the Meetup Group). In your message, tell your members that they should join Meetup Group 1 and then remove themselves from Meetup Group 2 in order for it to be closed.
When you are the only member left in Meetup Group 2, get in touch with the Meetup Community Team, and we'll remove it from the site.