I manually type the "!"-prefixed group name each time I post something into it. And that is a reasonable approach in !gs Gnusocial because that way everybody, be he member of that #group or not, can post into this group using the prefix and the group name. Of course, one needs to be a member of the group to get the group notifications lifted into one's Home TL. And the admin of the group (me, in this case) can block accounts from sending posts to that group (as far as I know blocking is only possible with regard to group members not non-group members). Anyway, one could write a script, I suppose, or just have the prefix and group name written down, e.g., in a notepad file from which one easily copy and paste it.