Is this happening for all users?
What have you upgraded, GoldMine is it? From which version to which version?
I was curious - not sure what version you are on, but we're on v2018.1. I get the same behavior when I right-ciick on the group name or click the Add Member button - both in the Contact Group window, so the issue can be recreated.
My only suggestion, as an alternative, is to perform a search for the contact(s) you want to add and then click the "Add to" button on the Search Center's toolbar and select Contact Group. And you know you can tag records, even if you have to perform a different search for each contact - enter a name in the search field, tag them when they appear - be sure not to double-click on the search result. Enter a new search name and when that result displays, tag that contact and so on. When you are done tagging all the records, you can click on the Add to button and select Contact Group.
Hope that helps. Good catch on the little bug.
I upgraded our system to 2018.1.0.65149 and can reproduce (was unable to reproduce in 2017.1) so it looks like a bug alright.
I have upgraded to 2018.1.0.261 from 2017.1. Thanks for the suggestion, however I use different history activity codes for email campaigns so now I have to create multiple groups and combine the contacts in excel.
Hope it gets fixed.