This doesn't appear in Elgg 3.3.16 with the bundled plugins.

Disable your 3rd party activated plugins and check your site again.

Hm...

1. Fresh installation Elgg 3.3.16 and login as admin.
2. Deactivate plugins: Friends, Friend Collections, Invite Friends.
3. Create the group: group1
4. Go menu Goups, then click the link "group1", go to the right bottom and click "View all members"
5. "Invite friends" stays at the up right corner (as reported before!)

Active plugins: Activity Pages, Blog, Bookmarks, CKEditor, Discussions, Embed, File, Garbage Collector, Groups, Likes, Members, Message Board, Messages, Notifications, Pages, Profile, Reported Content, Search, System Log, The Wire, User Validation by Email.

Inactive plugins: Front Page Demo, User Dashboard, Elgg Developer Tools, Diagnostics, Site Pages, Friends, Friend Collections, Invite Friends, Login As, Site Notifications, Tag Cloud, Web Services.

Non-bundled plugins: Friends (but Deactived).

Confirmed.

PR is added.

https://stackoverflow.com/questions/53020833/count-parameter-must-be-an-array-or-an-object-that-implements-countable-in-lar

Nope.

You should create a custom plugin for this.

Learn more on http://learn.elgg.org/en/3.x/guides/routing.html

But there's Static plugin that lets you use custom pages with your URI.

Static plugin is ok - great!