What can I call the different type of groups in this scenario?

Question

Local volunteers can join certain groups to contribute their services, and the rules are:

1. a volunteer must choose one group as the initial group, and he serves in this group most of the time

2. a volunteer can join multiple non-initial groups according to their interests, but he only serves in those groups in spare time

Now I'd like to find two words to identify the initial group & non-initial group.

How can I express this?

(here's some examples that I don't know if ok)

1. Primary Group & Secondary Group
2. Major Group & Minor Group
3. ....

Answer 1

There is a number of ways you can name the groups.

1 would be correct, as the groups are primary and secondary to a certain volunteer, but I don't think 2 is a good option: major and minor suggest size instead of priority, and the fact that the volunteer doesn't consider the second group as his/her main group doesn't make it smaller.

It's important to consider that the names you choose are in relation to the volunteer, rather to the groups themselves.

Some other examples I can think of are:

1. Main group and secondary groups
2. Main group and additional groups
3. Main group and minor groups

Any of those replacing Main with Head or Primary would also be correct.