"an existing member" is correct and usable in written English.
It can be used to refer to someone who is already a part of a particular group or organization. For example, "All existing members of the club were invited to the annual meeting."
Each has been tapped, in Skull and Bones fashion, by an existing member.
In one way the membership of the society is self-perpetuating: new members must be nominated by an existing member.
"Having a glorified suggestion box or giving that responsibility to an existing member of the board does not amount to a genuine voice for workers.
And, one way or another, Scotland should continue to benefit from European research funding – if not as a member state (because Scotland, seceding from an existing member state, may have to apply for entry) then as an associated country.
For most of the NYSE's history, ownership of the exchange was controlled by members limited to 1,366 (since 1953)—and the only means of obtaining a membership was by purchasing (since 1868) a seat from an existing member.
It could be an existing member of staff or someone external; what is crucial is that, at a very minimum, pupils should have had at least one guidance interview by the age of 16 (and I'd argue at least another one by 18).
Others, such as Squa.re and Quintessentially, are more lenient, requiring simply an invite from any existing member.
Then the state of affairs of Obama being accompanied would need to be identical to the state of affairs of there being a contingently existing member of C that Obama coexists with and is wholly distinct from.
The network can also provide for mobilizing supporters or associates on regional basis – with one or a group of existing member organizations in the relevant region providing for coordination of the regional efforts.
For existing member states, a concrete Roma policy should set similar indicators for success.
When a new sibling arrives in a family, the existing members often get pretty excited about it.
