1. If both speakers are adults, this is supposed to be symmetric, so you can wait for the others.

2. Unfortunately, native speakers have more practice to avoid the issue with complicated avoidance maneuvers in awkward social situations, so you might just want to ask and use your foreigner status as good excuse for asking.

3. The younger the people, the more they use the informal address. Especially among students or colleagues.

4. In written emails, one might write something like:

Lieber Herr Wheeler! Ich freue mich, dass Sie mich kontaktieren ....

Schöne Grüße, Susi

(The first name signature indicates a willingness to switch to a more informal address and a shyness to actually ask.)

Liebe Susi!

Ich darf doch du sagen? ....

Liebe Grüße, Glen

.......

Or, in an email to the other participants of your new choir:

Liebe MitsängerInnen! (or Liebe Mitsänger! or Liebe Mitsängerinnen und Mitsänger! or Lieber Chor! if you want to avoid this dividing issue)

Ich hoffe, dass niemand etwas dagegen hat, dass wir per du sind. Ich wollte euch alle zu meiner Gartenparty Ende Juni einladen ....

Note that traditionally there have been strict etiquette rules on who can offer the informal address to whom (e.g. always the older one to the younger or always the woman to the man), but 19th century etiquette is rarely enforced these days. But it is still a matter of power and respect and you should not take the initiative if you are in a clearly subordinate position.

1. If both speakers are adults, this is supposed to be symmetric, so you can wait for the others.

2. Unfortunately, native speakers have more practice to avoid the issue with complicated avoidance maneuvers in awkward social situations, so you might just want to ask and use your foreigner status as good excuse for asking.

3. The younger the people, the more they use the informal address. Especially among students or colleagues.

4. In written emails, one might write something like:

Lieber Herr Wheeler!
Ich freue mich, dass Sie mich kontaktieren ....

Schöne Grüße 
Susi

(The first name signature indicates a willingness to switch to a more informal address and a shyness to actually ask.)

Liebe Susi!

Ich darf doch du sagen? 
....

Liebe Grüße 
Glen

.......

Or, in an email to the other participants of your new choir:

Liebe MitsängerInnen!

(or Liebe Mitsänger! or Liebe Mitsängerinnen und Mitsänger! or Lieber Chor! if you want to avoid this dividing issue)

Ich hoffe, dass niemand etwas dagegen hat, dass wir per du sind.
Ich wollte euch alle zu meiner Gartenparty Ende Juni einladen ....

Note that traditionally there have been strict etiquette rules on who can offer the informal address to whom (e.g. always the older one to the younger or always the woman to the man), but 19th century etiquette is rarely enforced these days. But it is still a matter of power and respect and you should not take the initiative if you are in a clearly subordinate position.